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.test.ts

@@ -1,5 +1,34 @@
 import { describe, it, expect } from "vitest";
-import { resolveVideoSource } from "./localSource";
+import { downloadedFilePath, resolveVideoSource } from "./localSource";
+
+describe("downloadedFilePath", () => {
+  // The download worker rewrites `downloads.file_path` to the absolute path it
+  // actually wrote once the transfer completes, so a completed row is already
+  // rooted. Joining it onto the storage root again produced
+  // `/data/user/0/app//data/user/0/app/videos/x.mp4`, which the asset protocol
+  // cannot open — offline video died with MEDIA_ERR_SRC_NOT_SUPPORTED while
+  // audio, which resolves the same column through Rust, played fine.
+  it("leaves a completed download's absolute path alone", () => {
+    const root = "/data/user/0/com.dtourolle.jellytau";
+    const stored = `${root}/videos/Taming of the Shrew.mp4`;
+
+    expect(downloadedFilePath(root, stored)).toBe(stored);
+  });
+
+  it("roots a path that is still relative to the storage directory", () => {
+    // Rows only hold a relative path before the worker completes them, but a
+    // half-migrated database can still carry one.
+    expect(downloadedFilePath("/var/data/jellytau", "videos/film.mp4")).toBe(
+      "/var/data/jellytau/videos/film.mp4"
+    );
+  });
+
+  it("leaves an absolute Windows path alone", () => {
+    const stored = "C:\\Users\\u\\AppData\\jellytau\\videos\\film.mp4";
+
+    expect(downloadedFilePath("C:\\Users\\u\\AppData\\jellytau", stored)).toBe(stored);
+  });
+});
 
 // A stand-in for Tauri's convertFileSrc, so the module stays pure.
 const toAssetUrl = (p: string) => `asset://localhost/${encodeURIComponent(p)}`;