E frontend reconciliation (2/2): align consumers to generated bindings; svelte-check clean

Reconcile the ~50 frontend files that consumed the old hand-written types against
the stricter generated bindings (141 -> 0 svelte-check errors):

- Nullable strictness: null-coalesce/guard at consumer sites; widen shared helpers
  (CachedImage.tag, ticksToSeconds, formatDuration, getSessionIcon, session store
  send*/transferToRemote) to accept null.
- Field-name fixes exposed by codegen: userData.played -> isPlayed; remote
  NowPlayingItem uses album (not albumName); ArtistItem id/name.
- player.ts: normalize remote NowPlayingItem -> MediaItem in mergedMedia.
- ArtistItem now serializes camelCase (PascalCase alias retained for Jellyfin
  deserialize); update its serialize test.
- Update downloads.test.ts to the bundled { request } shape; complete the
  sessions.test.ts NowPlayingItem mock.

Frontend: svelte-check 0 errors, vitest 448 passing. Backend: 387 passing.

src/lib/stores/playbackMode.ts

@@ -73,7 +73,7 @@ function createPlaybackModeStore() {
    * - Polls remote session until track loads
    * - Stops local playback
    */
-  async function transferToRemote(sessionId: string): Promise<void> {
+  async function transferToRemote(sessionId: string | null | undefined): Promise<void> {
     console.log("[PlaybackMode] Transferring to remote session:", sessionId);
     update((s) => ({ ...s, isTransferring: true, transferError: null }));
 
@@ -100,11 +100,11 @@ function createPlaybackModeStore() {
       }
 
       // Update local state
-      sessions.selectSession(sessionId);
+      sessions.selectSession(sessionId ?? null);
       update((s) => ({
         ...s,
         mode: "remote",
-        remoteSessionId: sessionId,
+        remoteSessionId: sessionId ?? null,
         isTransferring: false,
       }));