feat(web): multi-track audiobook player — per-track src switching, global-seconds position

The audio reader pointed <audio src> at ONE file — correct for m4b but
impossible for folder-of-tracks and zip books (the element holds a
single source). The player now generalizes to a track list built from
the /chapters manifest: single-file books are exactly one track whose
url is the whole file (zero behavior change — same element, same URL
flow), multi-file books swap src at track boundaries.

Mapping helpers (exported, unit-testable): trackIndexFor resolves a
global position to the owning track, localOffsetFor/globalSecondsFor
convert between global seconds — the coordinate system of chapters,
bookmarks and audio_offset_seconds — and (track, local offset). Every
consumer keeps global semantics: scrubber, skip ±N, bookmark saves,
?loc=audio: deep links and progress pushes are byte-identical payloads
server-side; the server contract does not move.

Mechanics: setActiveTrack swaps src and queues the position for
loadedmetadata (setting currentTime before the new metadata loads
resets it); ended advances to the next track and keeps playing;
timeupdate reports globalPosition(); playbackRate/volume re-apply
after swaps; src is deferred until the manifest loads (data-file-url
is a directory/zip root for multi-file books, not playable) with the
catch branch falling back to it for offline single-file starts.

web/src/audio.ts

@@ -18,6 +18,36 @@ import { showToast } from "./toast";
 
 type AudioChapter = { index: number; title: string; start: number; end: number };
 
+// One playable file of an audiobook. Multi-file books (folder/zip
+// layouts) carry several; single-file books carry exactly one spanning
+// the whole file. start/end are GLOBAL seconds — the same coordinate
+// system as chapters, bookmarks and audio_offset_seconds.
+export type AudioTrack = { url: string; start: number; end: number; title?: string };
+
+// trackIndexFor finds the track holding a global position: the last one
+// whose start is at or before it (zero-duration/unknown-end tracks keep
+// working because only start matters for membership).
+export function trackIndexFor(tracks: AudioTrack[], globalSeconds: number): number {
+  for (let i = tracks.length - 1; i > 0; i -= 1) {
+    if (globalSeconds >= tracks[i].start) return i;
+  }
+  return 0;
+}
+
+// localOffsetFor converts a global position to seconds within track i.
+export function localOffsetFor(tracks: AudioTrack[], index: number, globalSeconds: number): number {
+  const track = tracks[index];
+  if (!track) return Math.max(0, globalSeconds);
+  return Math.max(0, globalSeconds - track.start);
+}
+
+// globalSecondsFor converts (track i, local seconds) back to the global
+// position — the inverse of localOffsetFor.
+export function globalSecondsFor(tracks: AudioTrack[], index: number, localSeconds: number): number {
+  const track = tracks[index];
+  return (track ? track.start : 0) + Math.max(0, localSeconds);
+}
+
 const PROGRESS_INTERVAL_MS = 30_000;
 
 interface AudioConfig {
@@ -57,6 +87,17 @@ Alpine.data("audioReader", () => ({
   scrubbing: false,
   scrubValue: 0,
 
+  // Track manifest: one AudioTrack per playable file. Single-file books
+  // (m4b/mp3) carry exactly one track whose URL is the whole file, so
+  // every code path below can treat playback as (track, local seconds)
+  // and map through globalSecondsFor/localOffsetFor.
+  fileUrl: "",
+  tracks: [] as AudioTrack[],
+  activeTrack: 0,
+  // Local seconds to apply to the <audio> element once the freshly
+  // switched track's metadata loads (src swaps reset currentTime).
+  pendingSeek: null as number | null,
+
   // Bookmarks (A2d): time-positioned, with notes. Audio locator
   // convention: position = "audio:<seconds>" (chapter_number filled for
   // display); dedup + cross-device sync happen server-side.
@@ -105,17 +146,19 @@ Alpine.data("audioReader", () => ({
       document.title = "REJ: " + (e.reason?.message || String(e.reason));
     });
     const body = document.body as HTMLElement;
-    const config = readConfig();
     const mediaId = body.getAttribute("data-media-item-id") || "";
     const title = body.getAttribute("data-title") || "";
     const cover = body.getAttribute("data-cover") || "";
     const fileUrl = body.getAttribute("data-file-url") || "";
     this.mediaItemId = mediaId;
     this.title = title;
+    this.fileUrl = fileUrl;
 
     const audio = this.audio();
     if (!audio) return;
-    audio.src = fileUrl;
+    // src is set by restorePosition once the track manifest is known —
+    // for multi-file books data-file-url (a directory or a zip root) is
+    // NOT directly playable. The catch branch below falls back to it.
     audio.playbackRate = Number(this.speed) || 1;
     audio.preload = "auto";
     // Restored volume/mute from the settings strip's persisted state.
@@ -145,8 +188,17 @@ Alpine.data("audioReader", () => ({
     }
 
     // Element events.
+    audio.addEventListener("loadedmetadata", () => {
+      // A freshly switched track applies its queued position once the
+      // browser knows the duration; same-track seeks set currentTime
+      // directly in setActiveTrack.
+      if (this.pendingSeek != null) {
+        audio.currentTime = Math.max(0, this.pendingSeek);
+        this.pendingSeek = null;
+      }
+    });
     audio.addEventListener("timeupdate", () => {
-      if (!this.scrubbing) this.currentTime = audio.currentTime;
+      if (!this.scrubbing) this.currentTime = this.globalPosition();
       this.tickChapter();
       // Honest session accounting: only ACTUAL PLAYING time counts. The
       // prevSnapshot walk keeps internal seeks/crossfades from inflating
@@ -158,6 +210,13 @@ Alpine.data("audioReader", () => ({
       }
       this.prevTick = now;
     });
+    audio.addEventListener("ended", () => {
+      // Natural end of track n: roll into n+1 and keep playing. At the
+      // last track the book is done — nothing to advance into.
+      if (this.activeTrack < this.tracks.length - 1) {
+        this.setActiveTrack(this.activeTrack + 1, 0, true);
+      }
+    });
     audio.addEventListener("play", () => {
       this.playing = true;
       this.prevTick = performance.now();
@@ -199,6 +258,53 @@ Alpine.data("audioReader", () => ({
     return document.getElementById("audio-element") as HTMLAudioElement | null;
   },
 
+  // ---- track manifest (multi-file books) ----
+
+  buildTracks(data: Record<string, unknown>) {
+    const raw = (data.tracks || []) as Array<Record<string, unknown>>;
+    if (raw.length > 0) {
+      this.tracks = raw.map((t) => ({
+        url: String(t.url || ""),
+        start: Number(t.start) || 0,
+        end: Number(t.end) || 0,
+        title: t.title ? String(t.title) : undefined,
+      }));
+    } else {
+      // Single-file book: the whole file is the one track. end stays 0
+      // (unknown) — the browser's own duration governs the element and
+      // membership never needs it for a single track.
+      this.tracks = [{ url: this.fileUrl, start: 0, end: 0 }];
+    }
+    this.activeTrack = 0;
+  },
+
+  globalPosition(): number {
+    const audio = this.audio();
+    if (!audio) return this.currentTime;
+    return globalSecondsFor(this.tracks, this.activeTrack, audio.currentTime);
+  },
+
+  // setActiveTrack points the <audio> element at track i at localSeconds.
+  // Same-track moves set currentTime directly; a src swap queues the
+  // position for loadedmetadata (setting currentTime before the new
+  // metadata loads throws or silently resets).
+  setActiveTrack(index: number, localSeconds: number, resume: boolean) {
+    const audio = this.audio();
+    if (!audio || index < 0 || index >= this.tracks.length) return;
+    const track = this.tracks[index];
+    this.activeTrack = index;
+    if (audio.getAttribute("src") === track.url) {
+      this.pendingSeek = null;
+      audio.currentTime = Math.max(0, localSeconds);
+    } else {
+      this.pendingSeek = Math.max(0, localSeconds);
+      audio.setAttribute("src", track.url);
+      audio.playbackRate = Number(this.speed) || 1;
+      audio.load();
+    }
+    if (resume) audio.play().catch(() => showToast("Playback failed to start", "error"));
+  },
+
   async restorePosition() {
     try {
       const [chaptersResp, progress] = await Promise.all([
@@ -212,6 +318,11 @@ Alpine.data("audioReader", () => ({
         const data = await chaptersResp.json();
         this.chapters = (data.chapters || []) as AudioChapter[];
         this.bookDuration = data.duration_seconds || this.chapterEndEstimate();
+        this.buildTracks(data);
+        const audio = this.audio();
+        if (audio && !audio.getAttribute("src") && this.tracks[0]) {
+          audio.setAttribute("src", this.tracks[0].url);
+        }
       }
       if (progress.ok) {
         const p = await progress.json();
@@ -233,7 +344,13 @@ Alpine.data("audioReader", () => ({
         this.tickChapter();
       }
     } catch {
-      // Offline start: still playable; sync resumes when reachable.
+      // Offline start: still playable from the plain file URL (single-
+      // file books only; multi-file books need the manifest).
+      const audio = this.audio();
+      if (audio && !audio.getAttribute("src") && this.fileUrl) {
+        this.tracks = [{ url: this.fileUrl, start: 0, end: 0 }];
+        audio.setAttribute("src", this.fileUrl);
+      }
     }
   },
 
@@ -282,14 +399,16 @@ Alpine.data("audioReader", () => ({
     const audio = this.audio();
     if (!audio) return;
     const target = Number(seconds);
-    if (Number.isFinite(target)) audio.currentTime = Math.max(0, target);
-    this.currentTime = audio.currentTime;
+    if (!Number.isFinite(target)) return;
+    // Global seconds in: resolve the owning track and move within it.
+    const index = trackIndexFor(this.tracks, target);
+    this.setActiveTrack(index, localOffsetFor(this.tracks, index, target), false);
+    this.currentTime = target;
   },
 
   skip(direction: 1 | -1) {
-    const audio = this.audio();
-    if (!audio) return;
-    audio.currentTime = Math.max(0, audio.currentTime + direction * (Number(this.skipSeconds) || 10));
+    const target = Math.max(0, this.globalPosition() + direction * (Number(this.skipSeconds) || 10));
+    this.seekTo(target);
   },
 
   stepChapter(delta: 1 | -1) {
@@ -406,7 +525,7 @@ Alpine.data("audioReader", () => ({
   async saveBookmark() {
     const audio = this.audio();
     if (!audio) return;
-    const at = Math.round(audio.currentTime);
+    const at = Math.round(this.globalPosition());
     const chapter = this.chapters[this.currentChapterIdx];
     const title = chapter
       ? `${chapter.title || `Chapter ${chapter.index + 1}`} — ${fmtTime(at)}`
@@ -502,7 +621,8 @@ Alpine.data("audioReader", () => ({
     if (this.chaptersDuration() <= 0) return;
     const audio = this.audio();
     if (!audio) return;
-    const at = audio.currentTime;
+    // Global book position, whatever track is physically playing.
+    const at = this.globalPosition();
     let chapter = 0;
     let offset = 0;
     for (let i = 0; i < this.chapters.length; i += 1) {