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/sessions.test.ts

@@ -40,8 +40,15 @@ describe("sessions store", () => {
     nowPlayingItem: {
       id: "item-1",
       name: "Test Song",
-      type: "Audio",
-      serverId: "server-1",
+      Type: "Audio",
+      runTimeTicks: null,
+      album: null,
+      albumId: null,
+      albumArtist: null,
+      artists: null,
+      imageTags: null,
+      primaryImageTag: null,
+      albumPrimaryImageTag: null,
     },
     playableMediaTypes: ["Audio", "Video"],
     supportedCommands: ["PlayPause", "Stop", "Seek", "NextTrack", "PreviousTrack"],