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/RemoteControls.svelte

@@ -15,8 +15,8 @@
   const nowPlaying = $derived(session.nowPlayingItem);
   const supportsSeek = $derived(playState?.canSeek ?? false);
   const supportsNextPrevious = $derived(
-    session.supportedCommands.includes("NextTrack") &&
-    session.supportedCommands.includes("PreviousTrack")
+    (session.supportedCommands ?? []).includes("NextTrack") &&
+    (session.supportedCommands ?? []).includes("PreviousTrack")
   );
 
   async function handlePlayPause() {
@@ -107,8 +107,8 @@
       <h4 class="text-base font-medium text-white truncate">{nowPlaying.name}</h4>
       {#if nowPlaying.artists && nowPlaying.artists.length > 0}
         <p class="text-sm text-gray-400 truncate">{nowPlaying.artists.join(", ")}</p>
-      {:else if nowPlaying.albumName}
-        <p class="text-sm text-gray-400 truncate">{nowPlaying.albumName}</p>
+      {:else if nowPlaying?.album}
+        <p class="text-sm text-gray-400 truncate">{nowPlaying?.album}</p>
       {/if}
     </div>
 
@@ -196,9 +196,9 @@
     {#if playState.volumeLevel !== undefined}
       <div class="flex items-center gap-3">
         <svg class="w-5 h-5 text-gray-400 flex-shrink-0" fill="currentColor" viewBox="0 0 24 24">
-          {#if playState.isMuted || playState.volumeLevel === 0}
+          {#if playState.isMuted || (playState.volumeLevel ?? 0) === 0}
             <path d="M16.5 12c0-1.77-1.02-3.29-2.5-4.03v2.21l2.45 2.45c.03-.2.05-.41.05-.63zm2.5 0c0 .94-.2 1.82-.54 2.64l1.51 1.51C20.63 14.91 21 13.5 21 12c0-4.28-2.99-7.86-7-8.77v2.06c2.89.86 5 3.54 5 6.71zM4.27 3L3 4.27 7.73 9H3v6h4l5 5v-6.73l4.25 4.25c-.67.52-1.42.93-2.25 1.18v2.06c1.38-.31 2.63-.95 3.69-1.81L19.73 21 21 19.73l-9-9L4.27 3zM12 4L9.91 6.09 12 8.18V4z" />
-          {:else if playState.volumeLevel < 50}
+          {:else if (playState.volumeLevel ?? 0) < 50}
             <path d="M7 9v6h4l5 5V4l-5 5H7z" />
           {:else}
             <path d="M3 9v6h4l5 5V4L7 9H3zm13.5 3c0-1.77-1.02-3.29-2.5-4.03v8.05c1.48-.73 2.5-2.25 2.5-4.02z" />