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/SessionCard.svelte

@@ -51,11 +51,11 @@
     <div class="flex items-center gap-3 mt-3 pt-3 border-t border-white/10">
       <div class="w-12 h-12 rounded overflow-hidden flex-shrink-0">
         <CachedImage
-          itemId={nowPlaying.id}
+          itemId={nowPlaying.id ?? ""}
           imageType="Primary"
           tag={nowPlaying.primaryImageTag}
           maxWidth={80}
-          alt={nowPlaying.name}
+          alt={nowPlaying.name ?? undefined}
           class="w-full h-full object-cover"
         />
       </div>
@@ -64,8 +64,8 @@
         <p class="text-sm font-medium text-white truncate">{nowPlaying.name}</p>
         {#if nowPlaying.artists && nowPlaying.artists.length > 0}
           <p class="text-xs text-gray-400 truncate">{nowPlaying.artists.join(", ")}</p>
-        {:else if nowPlaying.albumName}
-          <p class="text-xs text-gray-400 truncate">{nowPlaying.albumName}</p>
+        {:else if nowPlaying.album}
+          <p class="text-xs text-gray-400 truncate">{nowPlaying.album}</p>
         {/if}
 
         <div class="flex items-center gap-2 mt-1">