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/EpisodeRow.svelte

@@ -40,7 +40,7 @@
     if (!episode.userData || !episode.runTimeTicks) {
       return 0;
     }
-    return (episode.userData.playbackPositionTicks / episode.runTimeTicks) * 100;
+    return ((episode.userData.playbackPositionTicks ?? 0) / episode.runTimeTicks) * 100;
   });
 
   const duration = $derived(formatDuration(episode.runTimeTicks));
@@ -137,7 +137,7 @@
             {episode.name}
           </h3>
           <!-- Played indicator -->
-          {#if episode.userData?.played}
+          {#if episode.userData?.isPlayed}
             <svg class="w-4 h-4 flex-shrink-0 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"/>
             </svg>
@@ -164,10 +164,10 @@
           <VideoDownloadButton
             itemId={episode.id}
             itemName={episode.name}
-            seriesName={episode.seriesName}
-            seasonName={episode.seasonName}
-            episodeNumber={episode.indexNumber}
-            seasonNumber={episode.parentIndexNumber}
+            seriesName={episode.seriesName ?? undefined}
+            seasonName={episode.seasonName ?? undefined}
+            episodeNumber={episode.indexNumber ?? undefined}
+            seasonNumber={episode.parentIndexNumber ?? undefined}
             size="sm"
           />
         </div>