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/ui.js

@@ -57,6 +57,8 @@ export function showSetupError(m) {
 
 // ─── SETTINGS ───
 export function openSettings() {
+  // Lazy import to avoid circular dep — abs.js imports views.js
+  import('./abs.js').then(m => m.absOpenSettings());
   document.getElementById('s-url').value     = S.cfg.url || '';
   document.getElementById('s-user').value    = S.cfg.user || '';
   document.getElementById('s-pass').value    = '';