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/components/sessions/SessionPickerModal.svelte

@@ -63,8 +63,8 @@
     }
   }
 
-  function getSessionIcon(client: string): string {
-    const clientLower = client.toLowerCase();
+  function getSessionIcon(client: string | null | undefined): string {
+    const clientLower = (client ?? "").toLowerCase();
     if (clientLower.includes("tv") || clientLower.includes("roku") || clientLower.includes("android tv")) {
       return "tv";
     } else if (clientLower.includes("web") || clientLower.includes("chrome") || clientLower.includes("firefox")) {