feat(audio): auto-advance to next episode in source queue on track end

When a track reaches natural EOF (player alive, no stream error), play the
next episode from the source that started it — search results, show, or
Feed — and stop at the end of the list. A crashed/killed daemon or failed
stream never auto-advances.

- add audio-queue.ts: pure next/prev selection from the navigation source
- audio-player: expose getPlaybackError() to distinguish EOF from failure
- useAudio: finalizeTrackEnd(autoAdvance) wiring, re-selecting the current
  episode no longer reloads from stale saved progress
- tests: audio-queue units, auto-advance integration (real mpv + local
  WAVs over HTTP), backend re-select no-reload test

src/utils/audio-player.ts

@@ -82,6 +82,11 @@ export interface AudioBackend {
 	getPauseState(): Promise<boolean | undefined>;
 	/** True while the player process is running (regardless of pause). */
 	isAlive(): boolean;
+	/** Last playback error (end-file reason "error"), or null when the last
+	 *  track ended cleanly (or nothing has failed yet). Lets callers
+	 *  distinguish a natural end-of-file from a stream failure — a failed
+	 *  episode must not auto-advance the queue. */
+	getPlaybackError(): string | null;
 	dispose(): void;
 }
 
@@ -591,13 +596,22 @@ export class MpvBackend implements AudioBackend {
 		// play checks it and skips its own stale paused-load.
 		this._intentPlaying = true;
 		await this.runLoadExclusive(async () => {
-			// Fast path: this exact URL was PRELOADED paused (boot restore) —
-			// mpv has been buffering it since boot, so flipping pause off starts
-			// audio ~instantly. Re-acquire the start position only when it
-			// moved meaningfully since the preload (progress saved meanwhile).
-			if (this._loadedUrl === url && this._loadedPaused && !this._ended) {
+			// Same episode re-selected (Enter in a list, key-repeat, a
+			// second tap on the playing row): the file is ALREADY in the
+			// player. Reloading with start=<saved progress> would audibly
+			// skip BACK and repeat already-played audio (saved progress
+			// lags the live position by up to the 5s persist interval), so
+			// align in place instead:
+			//   - preload park (loaded paused at boot restore): seek only
+			//     when the caller's target moved materially since load;
+			//   - user-paused: unpause at the CURRENT position (saved
+			//     progress is stale and must not become a backward seek);
+			//   - already playing: unpause is a no-op — nothing to do.
+			// A genuinely finished episode (_ended) still falls through to
+			// a fresh load, which replays from the top via isCompleted.
+			if (this._loadedUrl === url && !this._ended) {
 				const target = opts?.startPosition ?? this._position;
-				if (Math.abs(target - this._position) > 2) {
+				if (this._loadedPaused && Math.abs(target - this._position) > 2) {
 					await this.send(["set_property", "time-pos", target]);
 					this._position = target;
 				}
@@ -782,6 +796,9 @@ class NoopBackend implements AudioBackend {
 	isAlive(): boolean {
 		return false;
 	}
+	getPlaybackError(): string | null {
+		return null;
+	}
 	dispose(): void {}
 }
 

src/utils/audio-queue.ts

@@ -0,0 +1,88 @@
+/**
+ * audio-queue — ordered episode queue for "what plays next" navigation.
+ *
+ * Pure selection logic for source-based auto-advance (and manual next/prev):
+ * given the navigation source that STARTED the current episode, which
+ * episodes come after it?
+ *
+ *   FEED      — the global chronological Feed list (newest first), so "next"
+ *               walks toward older episodes — further down the list.
+ *   MY_SHOWS  — the current show's episode list (newest first), scoped to the
+ *               podcast that started playback.
+ *   SEARCH    — the current search results, in display order (episode-kind
+ *               results only — a show result has nothing to play).
+ *
+ * Kept dependency-light (pure functions over plain data) so the ordering and
+ * bounds contract is unit-testable without stores or audio.
+ */
+
+import type { Episode } from "../types/episode";
+import type { Feed } from "../types/feed";
+import type { SearchResult } from "../types/source";
+import { AudioSource } from "../stores/audio-nav";
+
+/** The ordered playable queue for a navigation source. Empty when the
+ *  source's context is missing (no podcastId, no search results, no feeds). */
+export function queueForSource(
+	source: AudioSource,
+	podcastId: string | undefined,
+	feeds: Feed[],
+	allEpisodes: Array<{ episode: Episode; feed: Feed }>,
+	searchResults: SearchResult[],
+): Episode[] {
+	if (source === AudioSource.FEED) {
+		// Dedupe by episode id: the same episode can appear twice after a
+		// refresh merge or when two feeds list it — a duplicate would make
+		// next/auto-advance step onto the CURRENT episode and replay it.
+		const seen = new Set<string>();
+		const unique: Episode[] = [];
+		for (const e of allEpisodes) {
+			if (seen.has(e.episode.id)) continue;
+			seen.add(e.episode.id);
+			unique.push(e.episode);
+		}
+		return unique;
+	}
+	if (source === AudioSource.MY_SHOWS) {
+		const feed = feeds.find((f) => f.podcast.id === podcastId);
+		return feed ? feed.episodes : [];
+	}
+	if (source === AudioSource.SEARCH) {
+		return searchResults
+			.filter((r) => r.kind === "episode")
+			.map((r) => r.episode);
+	}
+	return [];
+}
+
+/** Index of an episode in the queue, or -1 when the episode isn't in it. */
+export function queueIndex(queue: Episode[], episodeId: string): number {
+	return queue.findIndex((e) => e.id === episodeId);
+}
+
+export interface QueueStep {
+	episode: Episode;
+	index: number;
+}
+
+/** The episode after `episodeId` in the queue, with its index. Null when
+ *  the episode isn't in the queue or is already the last one. */
+export function nextStep(
+	queue: Episode[],
+	episodeId: string,
+): QueueStep | null {
+	const idx = queueIndex(queue, episodeId);
+	if (idx < 0 || idx + 1 >= queue.length) return null;
+	return { episode: queue[idx + 1], index: idx + 1 };
+}
+
+/** The episode before `episodeId` in the queue, with its index. Null when
+ *  the episode isn't in the queue or is already the first one. */
+export function prevStep(
+	queue: Episode[],
+	episodeId: string,
+): QueueStep | null {
+	const idx = queueIndex(queue, episodeId);
+	if (idx <= 0) return null;
+	return { episode: queue[idx - 1], index: idx - 1 };
+}