fix(downloads): queue the whole album, and make every queued track findable offline

An album download put a handful of its tracks on the device while the button
reported the album as downloaded. Two independent gaps, one shared cause.

- `download_album` read its track list from `items WHERE album_id = ?` — the
  local catalog cache. Jellyfin does not return `AlbumId` on every listing
  endpoint, so tracks cached from one of those sit in `items` with a NULL
  `album_id` and are invisible to that query. On the reported database three
  whole albums (18, 12 and 9 tracks) had it NULL on every track; a partially
  linked album queued only the linked subset.
- The frontend then resolved one stream URL per track from its own list and
  paired it with the returned row ids by position. The ids came back in the
  backend's `index_number` order over a different set of rows, so a row could
  be handed another track's URL and any track past the end of the shorter list
  was never started. On Android that loop also stopped wherever the webview was
  suspended.
- `album_id` is what `OfflineRepository::get_items` joins a track to its album
  on, so a track that did download stayed invisible under its album offline —
  the same missing link seen from the other side.

The operation now belongs to Rust end to end:

- `HybridRepository::get_album_tracks` asks the server what the album contains.
  Cache-first `get_items` is right for browsing and wrong for deciding what to
  download; it errors offline so the caller falls back to the ungated local
  catalog, keeping the queue-while-offline flow.
- `queue_album_tracks` writes the album link onto every track it queues, and
  creates an `items` row for tracks the cache has never seen.
- Stream URLs resolve here, through the existing reconnect resolver, now scoped
  to the rows just queued so one album cannot start every unrelated pending row.
  Only the album id crosses the IPC boundary.
- `album_file_names` gives each track its own file. A title repeated inside one
  album (deluxe edition, two discs) mapped to one path, so those downloads
  overwrote each other.

Re-tapping download on a broken album heals it: missing tracks are queued and
the tracks already on disk get their link.

`download_series`/`download_season` still derive their episode lists from the
cache the same way and want the same treatment.

DR-173, UT-170..172. Rust 673 tests, frontend 975 tests, svelte-check and
check:boundary clean.

Note: this tree is shared with a concurrent session. Only the files above are
committed; docs/traceability.md is left to be regenerated once that work lands.

docs/requirements.md

@@ -327,6 +327,7 @@ Internal architecture, components, and application logic.
 | DR-167 | Each downloaded library shows only its own media. Cached items carry no link back to their library — `library_id` and `parent_id` are NULL on every row ([[offline-libraries-never-cached]]) — so `get_downloaded_items` matched the library branch with `EXISTS (SELECT 1 FROM libraries l WHERE l.id = ?)`, which asserts only that the requested library *exists* and never constrains the item to it. Opening any downloaded library therefore listed every downloaded top-level item on the server: films under Music, albums under TV. The sibling query that decides which libraries *appear* already carried the right rule — a `collection_type` ↔ `item_type` mapping — so the two disagreed about the same question. That mapping is now the named constant `LIBRARY_HOLDS_ITEM`, used by both, and a library of unknown collection type still keeps everything rather than being emptied by a rule that cannot classify it. The taxonomy stays in Rust, never the frontend | Downloads | UR-055 | Done |
 | DR-168 | Pause and resume actually stop and restart the bytes. `pause_download` wrote `status = 'paused'` and did nothing else, and no cancellation existed anywhere in the download stack — no token, no flag, no abort — so the streaming task ran on, kept writing, and overwrote the row with `completed`/`failed` when it finished: the row flicked to "paused" and undid itself. `resume_download` had the mirror defect, flipping the row to `pending` without calling `pump_download_queue`; the pump runs when something calls it rather than polling, so a resumed download sat untouched until an unrelated event happened to pump the queue. A per-download stop flag (`download::stop`) is the missing half — a module-level registry because the two sides never meet, the command holding Tauri state and the worker running detached in `async_runtime::spawn`. The worker reads it between chunks and on retry (so a pause is not swallowed by a 45-second backoff), flushes, and returns `Stopped`, which is deliberately **not** retryable and **not** recorded as a failure: the `.part` file is left intact because that is exactly what the resume's Range request continues from. Registering returns a *fresh* flag, or a resumed download would inherit the pause that stopped it and halt instantly. Cancel and `clear_stale_downloads` signal it too, so neither deletes a file still being written | Downloads | UR-055 | Done |
 | DR-169 | Partial files are actually reaped. The worker named its sidecar with `Path::with_extension("part")`, which *replaces* the extension — `movie.mp4` became `movie.part` — while every cleanup path deleted `"{file_path}.part"`, i.e. `movie.mp4.part`. The two never matched, so the partial file of every cancelled or failed download stayed on disk indefinitely, invisible to the disk-usage totals because no `downloads` row pointed at it. `partial_path` appends instead, is the single definition both the writer and the cleaners use, and incidentally removes a collision the old form had, where `movie.mp4` and `movie.mkv` mapped to one `movie.part` | Downloads | UR-055 | Done |
+| DR-173 | Downloading an album queues the **whole** album, and every track it queued is findable offline afterwards. Two independent gaps left an album with a handful of its tracks on the device while the button reported the album as downloaded. First, `download_album` took its track list from `items WHERE album_id = ?` — the local catalog cache. Jellyfin does not return `AlbumId` on every listing endpoint, so tracks cached by one of those endpoints sit in `items` with a NULL `album_id` and are invisible to that query; on the reporter's database three whole albums (18, 12 and 9 tracks) had it NULL on *every* track, so "download album" would have queued nothing for them, and a partially-linked album queued only the linked subset. Second, the frontend then resolved one stream URL per track from its own list and paired it with the returned row ids **by position** — a pairing with no basis, since the ids came back in the backend's `index_number` order over a different set of rows, so a row could be handed another track's URL and any track past the end of the shorter list was never started at all; on Android that loop also stopped wherever the webview was suspended. The same `album_id` is what `OfflineRepository::get_items` joins a track to its album on, so a track that did download stayed invisible under its album offline — the two halves of the same missing link. The operation now belongs to Rust end to end: `HybridRepository::get_album_tracks` asks the **server** what the album contains (cache-first `get_items` is right for browsing and wrong for deciding what to download) and errors offline so the caller falls back to the ungated local catalog, keeping the queue-while-offline flow; `queue_album_tracks` writes the album link onto every track it queues — queuing a track *is* the statement that it belongs to the album, rather than something to hope a listing endpoint recorded — and the stream URLs are resolved here through the existing reconnect resolver, now scoped to the rows just queued so one album cannot start every unrelated pending row. Nothing crosses the IPC boundary but the album id. Re-queuing a broken album heals it: the missing tracks are added and the tracks already on disk get their link. `download_series`/`download_season` still derive their episode lists from the cache the same way and want the same treatment | Downloads | UR-018, UR-055 | Done |
 | DR-170 | Downloads at a chosen bitrate are no longer corrupted by their own retries. Only the `original` preset asks for `Static=true`; every other rung requests a **transcode**, which Jellyfin serves chunked, with no `Content-Length`, and cannot byte-seek — so it ignores `Range` and answers `200` with the whole stream from the beginning rather than `206` with the requested tail. The worker sent the Range header whenever a `.part` existed and appended the body unconditionally, so each retry and each resume concatenated a fresh copy of the entire transcode onto the bytes already on disk: the file grew past its real size and would not play, which is why "downloads for different bitrates" stayed broken after the `videoBitRate` casing fix (DR-adc460f3) corrected the *request*. `resume_offset` makes the response decide — append only on a `206`, otherwise truncate and take the stream from the top — and the total size is computed from that offset rather than from a partial length the server never agreed to | Downloads | UR-071 | Done |
 | DR-172 | Native Android video is opt-in again, because as a default it shipped as **audio with no picture**. DR-161 flipped `experimentalNativeVideo` on so picture-in-picture could shrink a real video surface; on a device that produced sound and a blank screen. The decode path was never the problem — logcat showed ExoPlayer running (`Position update` ticks) and feeding a live `SurfaceView` with an active BufferQueue. The compositing was: the SurfaceView sits *behind* the WebView, and the step that clears the opaque layers above it never took effect, with `WebView transparent = false` logged and `= true` never appearing. So the video rendered correctly the whole time, behind an opaque page. This is exactly the defect the flag existed to contain — `VideoPlayer.scrubRegression.test.ts` had recorded that "the native SurfaceView has never been visible through the webview" — and enabling it by default shipped a verified decode path on top of an unverified display path. Reverting costs nothing that matters: PiP does not depend on it (DR-160 drives PiP from the WebView `<video>`), and working video outranks PiP showing a native surface. The flag stays available in Settings, now described as incomplete rather than as a performance win, and the scrub-regression mocks that were made explicit under DR-161 are kept explicit so those tests state which path they guard rather than inheriting a default that has now moved twice. Fixing the compositing is the prerequisite for trying this default again | UI | UR-003, UR-004, UR-041 | Done |
 | DR-171 | A downloaded video keeps audio the device can actually decode. `original` quality asked for `Static=true`, which hands back the source file byte-for-byte — E-AC-3/AC-3/DTS/TrueHD track included — and video is rendered on both platforms by the webview `<video>` element, which decodes none of them. Streaming already knew this: DR-149 judges the track the server would serve against `WEBVIEW_AUDIO_CODECS` and forces a transcode over Jellyfin's own direct-play offer, because 10.11.5 honours a `DirectPlayProfile`'s container and video codec but ignores its audio codec. The download path never consulted that policy, so the *same film* had sound when streamed and played as picture in silence once downloaded — and offline a download is the only source a video has, so there was no working path left to fall back to. The rule is now one rule: `served_audio_codec` picks the track the server will serve (the default, or the first when none is marked) and both callers judge it, the streaming verdict staying a bool and the download path needing the codec itself so it can say what to re-encode. Only the audio is re-encoded — `allowVideoStreamCopy=true` keeps an h264 source's picture byte-for-byte and no bitrate or resolution cap is added, so `original` still means original quality; a source the webview could not have rendered anyway (HEVC) becomes h264 as a side effect, which is the only form of it that would have played. The decision is per item rather than blanket because the transcode costs the byte-range resumability `Static=true` gives the download worker (see DR-170 for what a chunked, length-less response does to a resume), so a file whose audio already plays keeps the direct copy. An unknown codec — item not fetchable, or the server named none — changes nothing: the policy only ever *adds* a transcode, so it cannot make a working download worse. The codec set judged against is the **webview's**, not the platform's, even though DR-161 made ExoPlayer the Android default: `experimentalNativeVideo` is a user setting, a downloaded file outlives whatever it was set to when the file arrived, and the narrow list is the only one that holds on both sides of it — at the cost of a Dolby-licensed device re-encoding a track its ExoPlayer could have played. `resolve_video_download_url` is the single entrance for all three resolution sites (the frontend's per-item command, the bulk series/season enqueue, and the offline-queued resume), since the pure builder cannot look a codec up and a caller that forgets to is exactly how the silent downloads shipped. **Files already downloaded stay silent** — the bytes on disk are the wrong bytes and only a re-download replaces them | Downloads | UR-071, UR-004 | Done |
@@ -369,7 +370,7 @@ Internal architecture, components, and application logic.
 | UR-015 | - | DR-005, DR-020 |
 | UR-016 | - | - |
 | UR-017 | - | DR-014, DR-021 |
-| UR-018 | IR-013 | DR-015, DR-018 |
+| UR-018 | IR-013 | DR-015, DR-018, DR-173 |
 | UR-019 | IR-015 | DR-022 |
 | UR-020 | IR-016, IR-018 | DR-023 |
 | UR-021 | IR-016, IR-019 | DR-024 |
@@ -406,7 +407,7 @@ Internal architecture, components, and application logic.
 | UR-052 | IR-027 | DR-078, DR-079, DR-080, DR-143 |
 | UR-053 | IR-029 | DR-074 |
 | UR-054 | - | DR-075, DR-076, DR-077, DR-147 |
-| UR-055 | - | DR-081, DR-082, DR-083, DR-084, DR-167, DR-168, DR-169 |
+| UR-055 | - | DR-081, DR-082, DR-083, DR-084, DR-167, DR-168, DR-169, DR-173 |
 | UR-056 | - | DR-085 |
 | UR-057 | - | DR-086 |
 | UR-058 | - | DR-087, DR-142 |
@@ -578,6 +579,9 @@ Internal architecture, components, and application logic.
 | UT-152 | Caching a server result mirrors its watch position locally — including for an item carrying a position but no favourite flag — without inventing a row for an item the server reported no user data for, and without pulling a still-unsynced local position backwards | DR-155 | Done |
 | UT-162 | Each downloaded library lists only its own media: the music library shows the album and neither the film nor the series, the movie library only the film, the TV library only the series | DR-163 | Done |
 | UT-163 | `partial_path` appends rather than replacing the extension, so it matches what the cleanup paths delete, keeps two sources for one title apart, and still produces a sidecar for an extension-less target | DR-165 | Done |
+| UT-170 | `queue_album_tracks` queues a row for every track of the album — including tracks the cache holds without an `album_id` and tracks it has never seen at all — links each one to its album so offline browsing can find it, returns the row ids in track order, and is idempotent: re-queuing fills the gaps without duplicating rows or resetting a completed track. `cached_album_tracks` (the offline fallback) finds tracks by either album link and does not sweep in another album's | DR-173 | Done |
+| UT-171 | `resolve_pending_download_urls` restricted to a set of row ids resolves only those rows and leaves other pending rows untouched, and an empty id set resolves nothing rather than sweeping everything | DR-173 | Done |
+| UT-172 | `album_file_names` gives every track of an album its own file: a title repeated within the album (deluxe edition, two discs) is disambiguated by track number and item id instead of the second download overwriting the first, an unambiguous title keeps its own name, and path separators in a title are sanitised so a track cannot escape the album directory | DR-173 | Done |
 | UT-164 | `resume_offset` appends only when the server answered `206`; a `200` after a Range request restarts the file, because that body is the whole stream | DR-166 | Done |
 | UT-165 | A registered download starts unflagged, `signal` sets the flag its worker reads, signalling an unregistered id reports not-in-flight, `clear` forgets it, and re-registering drops a previous stop so a resumed download does not halt instantly | DR-164 | Done |
 | UT-166 | `original` quality re-encodes audio the webview cannot decode (E-AC-3/AC-3/DTS/TrueHD) to AAC without capping bitrate or resolution, keeps the `Static=true` direct copy for audio that plays here (AAC/MP3/Opus/Vorbis/FLAC) and for an unknown codec, leaves the explicit quality presets untouched, and picks the served track by the same default-or-first rule the streaming verdict uses | DR-171 | Done |