feat: Audiobookshelf integration — browse, stream, chapters, sleep timer

- src/abs.js: full ABS API client + player engine
  - Login/token auth, library browsing, search
  - Playback session API (multi-file book support via startOffset)
  - Progress sync to ABS every 30s and on pause
  - Chapter navigation (prev/next/seek-to), ±10s / ±30s skip
  - Speed control (0.75–2×), sleep timer with countdown
- index.html: sidebar nav, audiobooks/detail pages, full-screen player overlay,
  ABS settings section (URL + credentials + library picker)
- state.js: abs + absPlayer state objects
- views.js: navigate() handles 'audiobooks' and 'abs-book-detail'
- styles.css: book grid, detail page, player overlay, pickers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -45,4 +45,24 @@ export const S = {
 
   // Artist images cache
   artistImgCache: {},
+
+  // Audiobookshelf
+  abs: {
+    url: '', user: '', token: null, userId: null,
+    currentLibraryId: null,
+  },
+  absPlayer: {
+    open: false,
+    item: null,
+    _sessionId: null,
+    audioTracks: [],
+    trackIdx: 0,
+    chapters: [],
+    chapterIdx: -1,
+    playing: false,
+    currentTime: 0,
+    duration: 0,
+    speed: 1.0,
+    sleepRemaining: null,
+  },
 };