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/library/MediaCard.svelte

@@ -52,7 +52,7 @@
     if (!showProgress || !("userData" in item) || !item.userData || !item.runTimeTicks) {
       return 0;
     }
-    return (item.userData.playbackPositionTicks / item.runTimeTicks) * 100;
+    return ((item.userData.playbackPositionTicks ?? 0) / item.runTimeTicks) * 100;
   });
 
   const subtitle = $derived(() => {
@@ -112,7 +112,7 @@
     {/if}
 
     <!-- Played indicator -->
-    {#if "userData" in item && item.userData?.played}
+    {#if "userData" in item && item.userData?.isPlayed}
       <div class="absolute top-2 right-2">
         <svg class="w-5 h-5 text-[var(--color-jellyfin)]" fill="currentColor" viewBox="0 0 24 24">
           <path d="M9 16.17L4.83 12l-1.42 1.41L9 19 21 7l-1.41-1.41z"/>