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/LibraryListView.svelte

@@ -18,7 +18,7 @@
   }
 
   function getImageTag(item: MediaItem | Library): string | undefined {
-    return "primaryImageTag" in item ? item.primaryImageTag : ("imageTag" in item ? item.imageTag : undefined);
+    return "primaryImageTag" in item ? (item.primaryImageTag ?? undefined) : ("imageTag" in item ? (item.imageTag ?? undefined) : undefined);
   }
 
   function getSubtitle(item: MediaItem | Library): string {
@@ -44,7 +44,7 @@
     if (!showProgress || !("userData" in item) || !item.userData || !("runTimeTicks" in item) || !item.runTimeTicks) {
       return 0;
     }
-    return (item.userData.playbackPositionTicks / item.runTimeTicks) * 100;
+    return ((item.userData.playbackPositionTicks ?? 0) / item.runTimeTicks) * 100;
   }
 
   function getTrackNumber(item: MediaItem | Library): string {
@@ -61,7 +61,7 @@
     {@const duration = "runTimeTicks" in item ? formatDuration(item.runTimeTicks) : ""}
     {@const progress = getProgress(item)}
     {@const trackNum = getTrackNumber(item)}
-    {@const isPlayed = "userData" in item && item.userData?.played}
+    {@const isPlayed = "userData" in item && item.userData?.isPlayed}
     {@const downloadInfo = getDownloadInfo(item.id)}
     {@const isDownloaded = downloadInfo?.status === "completed"}
     {@const isDownloading = downloadInfo?.status === "downloading" || downloadInfo?.status === "pending"}