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.