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/lib/player/localSource.ts

@@ -36,6 +36,28 @@ export interface VideoSourceDecision {
   isLocal: boolean;
 }
 
+/** Absolute on POSIX (`/…`), Windows (`C:\…`, `C:/…`) or a UNC share (`\\…`). */
+function isAbsolute(path: string): boolean {
+  return path.startsWith("/") || path.startsWith("\\") || /^[A-Za-z]:[\\/]/.test(path);
+}
+
+/**
+ * The on-disk path of a row in the `downloads` store.
+ *
+ * `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. Joining
+ * it onto the storage root a second time produced
+ * `/data/user/0/app//data/user/0/app/videos/x.mp4`; the asset protocol could not
+ * open that, so offline video failed with `MEDIA_ERR_SRC_NOT_SUPPORTED` while
+ * audio, which resolves the same column through Rust, played fine.
+ *
+ * TRACES: UR-071 | DR-133 | UT-124
+ */
+export function downloadedFilePath(storageRoot: string, filePath: string): string {
+  return isAbsolute(filePath) ? filePath : `${storageRoot}/${filePath}`;
+}
+
 export function resolveVideoSource(inputs: VideoSourceInputs): VideoSourceDecision {
   const { localPath, remoteUrl, remoteNeedsTranscoding, toAssetUrl } = inputs;