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/utils/app-persistence.ts

@@ -219,9 +219,14 @@ export async function loadLastPlayerFromFile(): Promise<LastPlayerState | null>
 	}
 }
 
-/** Save the last-loaded-player marker (overwrite, fire-and-forget) */
+/** Serialized marker-write chain: concurrent writes land in submission
+ *  order, and callers can await the last one (tests read the file back
+ *  deterministically). Mirrors updateConfig's write serialization. */
+let lastPlayerWriteChain: Promise<void> = Promise.resolve();
+
+/** Save the last-loaded-player marker (fire-and-forget) */
 export function saveLastPlayerToFile(state: LastPlayerState): void {
-	(async () => {
+	lastPlayerWriteChain = lastPlayerWriteChain.then(async () => {
 		try {
 			await ensureConfigDir();
 			await Bun.write(
@@ -231,7 +236,12 @@ export function saveLastPlayerToFile(state: LastPlayerState): void {
 		} catch {
 			// Silently ignore write errors
 		}
-	})();
+	});
+}
+
+/** Resolves once every marker write submitted so far has landed on disk. */
+export function waitForLastPlayerWrite(): Promise<void> {
+	return lastPlayerWriteChain;
 }
 
 /** Synchronous variant for the process-exit teardown. `q` quits through