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/EpisodeFocusView.svelte

@@ -84,7 +84,7 @@
     return null;
   });
 
-  function formatDuration(ticks?: number): string {
+  function formatDuration(ticks?: number | null): string {
     if (!ticks) return "";
     const seconds = Math.floor(ticks / 10000000);
     const hours = Math.floor(seconds / 3600);
@@ -100,7 +100,7 @@
     if (!ep.userData || !ep.runTimeTicks) {
       return 0;
     }
-    return (ep.userData.playbackPositionTicks / ep.runTimeTicks) * 100;
+    return ((ep.userData.playbackPositionTicks ?? 0) / ep.runTimeTicks) * 100;
   }
 
   function handlePlay() {
@@ -178,7 +178,7 @@
               {episode.communityRating.toFixed(1)}
             </span>
           {/if}
-          {#if episode.userData?.played}
+          {#if episode.userData?.isPlayed}
             <span class="flex items-center gap-1 text-[var(--color-jellyfin)]">
               <svg class="w-4 h-4" 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"/>
@@ -281,7 +281,7 @@
             {/if}
 
             <!-- Played indicator -->
-            {#if ep.userData?.played}
+            {#if ep.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"/>