fix(player): play downloaded video offline (DR-133, DR-134)

Offline video never started: the <video> element reported NETWORK_NO_SOURCE
one millisecond after loadstart, which the UI mislabelled as "may need
transcoding" even though nothing had been fetched. Two independent causes,
both required for playback.

The path was doubled. `downloads.file_path` is stored relative to the storage
root while a download is queued, but the worker rewrites it to the absolute
path it actually wrote once the transfer completes — so a completed row is
already rooted. The player's offline branch rooted it a second time, producing
/data/user/0/app//data/user/0/app/videos/x.mp4. Audio was unaffected because it
resolves the same column through Rust's resolve_local_media_path, which does
not re-root. The join is now absolute-aware (POSIX, Windows drive letters, UNC)
so rows written before completion still resolve.

The asset protocol was never enabled. convertFileSrc rewrites a path to
http://asset.localhost/… unconditionally, but Tauri only answers that origin
when the protocol-asset cargo feature is compiled in *and*
app.security.assetProtocol.enable is set — neither was, so even a correct path
resolved to nothing. This also silently defeated the cached-thumbnail path in
imageCache, which fails soft to the server copy and so hid the breakage
whenever the server was reachable. Scoped to $APPDATA/** — the storage root
holding the database, downloads/ and the thumbnail cache — rather than an
unrestricted grant.

Diagnosed from logcat on device; UT-124 reproduces the doubled path.

src/routes/player/[id]/+page.svelte

@@ -4,7 +4,7 @@
   import { goto } from "$app/navigation";
   import { convertFileSrc } from "@tauri-apps/api/core";
   import { commands } from "$lib/api/bindings";
-  import { resolveVideoSource } from "$lib/player/localSource";
+  import { downloadedFilePath, resolveVideoSource } from "$lib/player/localSource";
   import type { PlayQueueRequest } from "$lib/api/bindings";
   import type { MediaItem, MediaKind } from "$lib/api/types";
   import { auth } from "$lib/stores/auth";
@@ -236,9 +236,12 @@
         console.log("loadAndPlay: Found local download, using offline playback:", localDownload.filePath);
         isOfflinePlayback = true;
 
-        // Get the storage path and construct full file path
+        // Get the storage path and resolve the file's location. A completed
+        // row already holds an absolute path (the worker rewrites it on
+        // completion), so it must not be rooted again — see downloadedFilePath.
+        // TRACES: UR-071 | DR-133
         const storagePath = await commands.storageGetPath();
-        const fullPath = `${storagePath}/${localDownload.filePath}`;
+        const fullPath = downloadedFilePath(storagePath, localDownload.filePath);
         console.log("loadAndPlay: Full local path:", fullPath);
 
         // Convert file path to asset URL that can be played in webview