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-tauri/src/repository/types.rs

@@ -405,15 +405,21 @@ mod tests {
 
     #[test]
     fn test_artist_item_serialize() {
-        // Test that ArtistItem serializes to PascalCase for consistency
+        // ArtistItem serializes to camelCase for the frontend; it still accepts
+        // Jellyfin's PascalCase on deserialize via #[serde(alias = ...)].
         let artist = ArtistItem {
             id: "test-id".to_string(),
             name: "Test Artist".to_string(),
         };
 
         let json = serde_json::to_string(&artist).expect("Failed to serialize");
-        assert!(json.contains(r#""Id":"test-id""#));
-        assert!(json.contains(r#""Name":"Test Artist""#));
+        assert!(json.contains(r#""id":"test-id""#));
+        assert!(json.contains(r#""name":"Test Artist""#));
+
+        let from_pascal: ArtistItem =
+            serde_json::from_str(r#"{"Id":"x","Name":"Y"}"#).expect("Failed to deserialize");
+        assert_eq!(from_pascal.id, "x");
+        assert_eq!(from_pascal.name, "Y");
     }
 
     #[test]