fix: ABS API key auth + mobile bottom-nav Hörbücher button

- Replace username/password login with API key (Bearer token) auth
  - New absVerifyApiKey() verifies via GET /api/me instead of POST /login
  - Settings shows single API Key field with hint (Profil → API Token in ABS)
  - API key never pre-filled in settings for security
- Add "Bücher" button to mobile bottom-nav (bnav-library), hidden until connected
- _showAbsNavItem() helper keeps sidebar + bnav in sync on connect/disconnect

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

src/main.js

@@ -19,7 +19,7 @@ import {
 } from './views.js';
 import { loadSpotifyImporter, resolveSpotifyLink, clearSpotifyPreview, spotifyDlAll } from './spotify.js';
 import {
-  absInitFromStorage, absOpenSettings, absConnectSettings, absSetLibrary, absDisconnect,
+  absInitFromStorage, absOpenSettings, absConnectSettings, absSetLibrary, absDisconnect, absVerifyApiKey,
   absPlayItem, absTogglePlay, absSeek, absSkipSecs, absNextChapter, absPrevChapter,
   absSetSpeed, absSleepIn, absOpenPlayer, absClosePlayer, absToggleChapters,
   absToggleSpeedPicker, absToggleSleepPicker, absSeekToChapter, absProgDown, absProgMove,
@@ -85,6 +85,8 @@ async function startApp() {
   absInitFromStorage();
   if (S.abs.token) {
     document.getElementById('nav-abs-section').style.display = '';
+    const bnav = document.getElementById('bnav-audiobooks');
+    if (bnav) bnav.style.display = '';
     absLoadSettingsLibraries();
   }
   if(_navLoad() && S._navCursor >= 0 && S._navStack[S._navCursor]) {