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/downloads.test.ts

@@ -80,14 +80,17 @@ describe("downloads store", () => {
       );
 
       expect(mockInvoke).toHaveBeenCalledWith("download_item", {
-        itemId: "item-1",
-        userId: "user-1",
-        filePath: "/path/to/file.mp3",
-        mimeType: "audio/mpeg",
-        priority: 10,
-        itemName: undefined,
-        artistName: undefined,
-        albumName: undefined,
+        request: {
+          itemId: "item-1",
+          userId: "user-1",
+          filePath: "/path/to/file.mp3",
+          mimeType: "audio/mpeg",
+          priority: 10,
+          itemName: null,
+          artistName: null,
+          albumName: null,
+          expectedSize: null,
+        },
       });
       expect(downloadId).toBe(123);
     });