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/CastSection.svelte

@@ -28,6 +28,7 @@
         continue;
       }
       const type = person.type;
+      if (!type) continue;
       if (type in groups) {
         groups[type].push(person);
       } else {
@@ -84,7 +85,7 @@
               <!-- Person image -->
               <div class="w-24 h-24 rounded-full overflow-hidden bg-[var(--color-surface)] mb-2">
                 <CachedImage
-                  itemId={person.id}
+                  itemId={person.id ?? ""}
                   imageType="Primary"
                   tag={person.primaryImageTag}
                   maxWidth={200}