feat(player): restore the last player session at boot

Reload the episode that was loaded in the player when the previous run
ended (persisted on play/load and synchronously at exit) into the Player
tab paused at its saved position — never autostarted. Episodes at or
above 98% completion are skipped, as are empty-player and unsubscribed
episodes.

- useAudio gains load(episode) (sets currentEpisode/position/Now Playing
  without starting the backend) and restoreLastSession(), triggered once
  at boot and serialized through a chain so a late-finishing boot restore
  can't clobber later state.
- togglePlayback branches on a startedPlayback flag: a restored episode
  starts the backend from the saved position; a paused one resumes.
- stop() clears the marker; the exit teardown writes it synchronously
  (process.exit bypasses async writes).
- feed/progress stores expose whenReady() so restore waits for the async
  boot loads; feed store gains findEpisode().
- app-persistence serializes last-player marker writes and exposes
  waitForLastPlayerWrite() for deterministic tests.
- tests/restore-session.test.ts: real modules + local RSS feed server;
  the real useAudio is imported via a ?restore-test query suffix to
  bypass the suite's mock.module leak across shared bun workers.

src/stores/progress.ts

@@ -53,11 +53,17 @@ async function initProgress(): Promise<void> {
 	setProgressMap(parsed);
 }
 
-// Fire-and-forget init
-initProgress();
+// Fire-and-forget init; the promise is exposed via whenReady() so boot-time
+// consumers (e.g. player-session restore) can await the file load.
+const progressInit = initProgress();
 
 function createProgressStore() {
 	return {
+		/**
+		 * Resolves once the persisted progress map has been loaded from disk.
+		 */
+		whenReady: () => progressInit,
+
 		/**
 		 * Get progress for a specific episode.
 		 */