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/utils/duration.ts

@@ -10,7 +10,7 @@
  * @param format Format type: "mm:ss" (default) or "hh:mm:ss"
  * @returns Formatted duration string or empty string if no ticks
  */
-export function formatDuration(ticks?: number, format: "mm:ss" | "hh:mm:ss" = "mm:ss"): string {
+export function formatDuration(ticks?: number | null, format: "mm:ss" | "hh:mm:ss" = "mm:ss"): string {
   if (!ticks) return "";
 
   // Jellyfin uses 10,000,000 ticks per second

src/lib/utils/playbackUnits.ts

@@ -25,8 +25,8 @@ export function secondsToTicks(seconds: number): number {
  * @param ticks - Time in Jellyfin ticks
  * @returns Time in seconds
  */
-export function ticksToSeconds(ticks: number): number {
-  return ticks / TICKS_PER_SECOND;
+export function ticksToSeconds(ticks: number | null | undefined): number {
+  return (ticks ?? 0) / TICKS_PER_SECOND;
 }
 
 /**