fix(downloads,playback): re-encode undecodable audio and carry the media source through

Work from a parallel session in the same working tree, committed here so the
branch is not left half-written. Attribution note: authored in a concurrent
Claude session, not by the author of the preceding commit.

- DR-171: a downloaded video keeps audio the device can actually decode.
  `original` quality asked for a straight copy, so an E-AC-3/AC-3/DTS/TrueHD
  track came down untouched and the webview had nothing to play it with.
- `get_video_download_url` gains the media source, so the URL is built against
  the source actually chosen rather than the item's default.
- Device profile and repository plumbing updated to match.

Verified green as a whole: 656 Rust tests, 945 frontend tests, svelte-check clean.

src-tauri/src/repository/mod.rs

@@ -197,14 +197,24 @@ pub trait MediaRepository: Send + Sync {
         format: &str,
     ) -> String;
 
-    /// Get video download URL (synchronous - just constructs URL)
-    /// Called by frontend via Tauri invoke (getVideoDownloadUrl in VideoDownloadButton.svelte)
+    /// Build the URL a video download is fetched from. Synchronous — it only
+    /// constructs a URL, so it stays testable without a server. Reach it through
+    /// [`resolve_video_download_url`] rather than calling it directly.
+    ///
+    /// `source_audio_codec` is the codec of the audio track the server would
+    /// serve (see [`served_audio_codec`]); `None` when it is not known. At
+    /// `original` quality it decides whether the file can be copied byte-for-byte
+    /// or has to have its audio re-encoded on the way down — a downloaded file is
+    /// played back with no server in reach, so it has to be decodable *here*.
+    ///
+    /// TRACES: UR-071 | DR-171
     #[allow(dead_code)]
     fn get_video_download_url(
         &self,
         item_id: &str,
         quality: &str,
         media_source_id: Option<&str>,
+        source_audio_codec: Option<&str>,
     ) -> String;
 
     /// Mark item as favorite
@@ -323,3 +333,44 @@ pub trait MediaRepository: Send + Sync {
         new_index: u32,
     ) -> Result<(), RepoError>;
 }
+
+/// The audio codec the server would serve for `item_id` — the default track, or
+/// the first when none is marked, matching the track Jellyfin picks.
+///
+/// `None` when the item has no audio, names no codec, or cannot be fetched. A
+/// caller must read that as "unknown", never as "fine": it is the input to a
+/// policy that only *adds* a transcode, so an unknown codec leaves behaviour
+/// exactly as it was.
+///
+/// TRACES: UR-071 | DR-171 | UT-166
+pub async fn served_audio_codec(repo: &dyn MediaRepository, item_id: &str) -> Option<String> {
+    let item = repo.get_item(item_id).await.ok()?;
+    let audio: Vec<(Option<&str>, bool)> = item
+        .media_streams
+        .as_deref()
+        .unwrap_or_default()
+        .iter()
+        .filter(|s| s.stream_type == "Audio")
+        .map(|s| (s.codec.as_deref(), s.is_default))
+        .collect();
+
+    device_profile::served_audio_codec(&audio).map(str::to_string)
+}
+
+/// Resolve the download URL for a video, applying the audio-codec policy that
+/// keeps the saved file playable offline (DR-171).
+///
+/// Every video download goes through here rather than calling the builder
+/// directly: the builder is pure and cannot look the codec up, and a caller that
+/// forgets to is exactly how the silent downloads shipped.
+///
+/// TRACES: UR-071 | DR-171
+pub async fn resolve_video_download_url(
+    repo: &dyn MediaRepository,
+    item_id: &str,
+    quality: &str,
+    media_source_id: Option<&str>,
+) -> String {
+    let codec = served_audio_codec(repo, item_id).await;
+    repo.get_video_download_url(item_id, quality, media_source_id, codec.as_deref())
+}