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

tests/audio-queue.test.ts

@@ -0,0 +1,155 @@
+/**
+ * audio-queue unit tests — pure selection logic for next/prev navigation
+ * and source-based auto-advance. Covers ordering, bounds, and the
+ * deduplication that prevents "next" from replaying the current episode.
+ */
+import { test, expect } from "bun:test";
+import {
+	queueForSource,
+	queueIndex,
+	nextStep,
+	prevStep,
+} from "../src/utils/audio-queue";
+import { AudioSource } from "../src/stores/audio-nav";
+import type { Episode } from "../src/types/episode";
+import type { Feed } from "../src/types/feed";
+import { FeedVisibility } from "../src/types/feed";
+import type { SearchResult } from "../src/types/source";
+
+function ep(id: string, n: number): Episode {
+	return {
+		id,
+		podcastId: "pod-" + id,
+		title: `Episode ${n}`,
+		description: "",
+		audioUrl: `https://example.com/${id}.mp3`,
+		duration: 600,
+		pubDate: new Date(2026, 0, n),
+	};
+}
+
+function feed(id: string, episodes: Episode[]): Feed {
+	return {
+		id,
+		podcast: {
+			id,
+			title: "Feed " + id,
+			description: "",
+			feedUrl: `https://example.com/${id}.xml`,
+			lastUpdated: new Date(),
+			isSubscribed: true,
+		},
+		episodes,
+		visibility: FeedVisibility.PUBLIC,
+		sourceId: "rss",
+		lastUpdated: new Date(),
+		isPinned: false,
+	};
+}
+
+function episodeResult(episode: Episode): SearchResult {
+	return {
+		sourceId: "itunes",
+		kind: "episode",
+		podcast: {
+			id: episode.podcastId,
+			title: "Show " + episode.podcastId,
+			description: "",
+			feedUrl: `https://example.com/${episode.podcastId}.xml`,
+			lastUpdated: new Date(),
+			isSubscribed: false,
+		},
+		episode,
+	};
+}
+
+const e1 = ep("e1", 1);
+const e2 = ep("e2", 2);
+const e3 = ep("e3", 3);
+
+test("FEED queue is the chronological global list, newest first", () => {
+	const f1 = feed("f1", [e3, e2]);
+	const f2 = feed("f2", [e1]);
+	const queue = queueForSource(
+		AudioSource.FEED,
+		undefined,
+		[f1, f2],
+		[
+			{ episode: e3, feed: f1 },
+			{ episode: e2, feed: f1 },
+			{ episode: e1, feed: f2 },
+		],
+		[],
+	);
+	expect(queue.map((e) => e.id)).toEqual(["e3", "e2", "e1"]);
+	expect(queueIndex(queue, "e2")).toBe(1);
+	expect(nextStep(queue, "e2")?.episode.id).toBe("e1");
+	expect(prevStep(queue, "e2")?.episode.id).toBe("e3");
+	expect(nextStep(queue, "e1")).toBeNull();
+	expect(prevStep(queue, "e3")).toBeNull();
+});
+
+test("FEED queue dedupes repeated episode ids (same episode listed twice)", () => {
+	// The same episode appears twice in the global list (e.g. a refresh
+	// merge duplicated a feed's entries). Without dedupe, nextStep after
+	// e2 would step onto e2 AGAIN — replaying the current episode.
+	const f1 = feed("f1", [e3, e2, e2, e1]);
+	const queue = queueForSource(
+		AudioSource.FEED,
+		undefined,
+		[f1],
+		[
+			{ episode: e3, feed: f1 },
+			{ episode: e2, feed: f1 },
+			{ episode: e2, feed: f1 },
+			{ episode: e1, feed: f1 },
+		],
+		[],
+	);
+	expect(queue.map((e) => e.id)).toEqual(["e3", "e2", "e1"]);
+	// Distinct objects sharing an id dedupe too.
+	const e2clone = { ...e2 };
+	const queue2 = queueForSource(
+		AudioSource.FEED,
+		undefined,
+		[f1],
+		[
+			{ episode: e3, feed: f1 },
+			{ episode: e2, feed: f1 },
+			{ episode: e2clone, feed: f1 },
+		],
+		[],
+	);
+	expect(queue2.map((e) => e.id)).toEqual(["e3", "e2"]);
+	expect(nextStep(queue2, "e2")).toBeNull(); // no self-step
+});
+
+test("MY_SHOWS queue scopes to the podcast that started playback", () => {
+	const fA = feed("podA", [e3, e2]);
+	const fB = feed("podB", [e1]);
+	const queue = queueForSource(
+		AudioSource.MY_SHOWS,
+		"podA",
+		[fA, fB],
+		[],
+		[],
+	);
+	expect(queue.map((e) => e.id)).toEqual(["e3", "e2"]);
+	// Unknown podcastId → empty queue (nothing to play next).
+	expect(
+		queueForSource(AudioSource.MY_SHOWS, "podX", [fA, fB], [], []),
+	).toEqual([]);
+});
+
+test("SEARCH queue filters to episode-kind results in display order", () => {
+	const queue = queueForSource(
+		AudioSource.SEARCH,
+		undefined,
+		[],
+		[],
+		[episodeResult(e1), episodeResult(e2)],
+	);
+	expect(queue.map((e) => e.id)).toEqual(["e1", "e2"]);
+	expect(queueIndex(queue, "e1")).toBe(0);
+	expect(queueIndex(queue, "e3")).toBe(-1);
+});