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>

index.html

@@ -531,9 +531,15 @@
           <span style="width:24px;height:24px;border-radius:6px;background:var(--green-dim);display:inline-flex;align-items:center;justify-content:center;font-size:12px">📚</span>
           Audiobookshelf
         </h3>
-        <div class="field"><label>Server URL</label><input id="abs-s-url" type="url" placeholder="https://books.example.com"></div>
-        <div class="field"><label>Benutzername</label><input id="abs-s-user" type="text" placeholder="admin"></div>
-        <div class="field"><label>Passwort</label><input id="abs-s-pass" type="password" placeholder="••••••••"></div>
+        <div class="field">
+          <label>Server URL</label>
+          <input id="abs-s-url" type="url" placeholder="https://books.example.com">
+        </div>
+        <div class="field">
+          <label>API Key</label>
+          <input id="abs-s-apikey" type="password" placeholder="Aus ABS: Profil → API Token">
+          <div class="hint">Einstellungen → Profil → API Token in Audiobookshelf</div>
+        </div>
         <div id="abs-lib-picker" style="display:none" class="field">
           <label>Bibliothek</label>
           <select id="abs-lib-select" onchange="absSetLibrary(this.value)" style="width:100%;padding:10px 12px;background:var(--bg2);border:1px solid var(--border2);border-radius:var(--radius-sm);color:var(--text);font-family:var(--font);font-size:14px;outline:none"></select>
@@ -895,6 +901,10 @@
         <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polygon points="12 2 15.09 8.26 22 9.27 17 14.14 18.18 21.02 12 17.77 5.82 21.02 7 14.14 2 9.27 8.91 8.26 12 2"/></svg>
         Favoriten
       </button>
+      <button class="bnav-item" id="bnav-audiobooks" onclick="navigate('audiobooks')" data-page="audiobooks" style="display:none">
+        <svg width="22" height="22" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M2 3h6a4 4 0 0 1 4 4v14a3 3 0 0 0-3-3H2z"/><path d="M22 3h-6a4 4 0 0 0-4 4v14a3 3 0 0 1 3-3h7z"/></svg>
+        Bücher
+      </button>
     </div>
     <div class="bnav-pages" id="bnav-discover" style="display:none">
       <button class="bnav-item" onclick="navigate('discover')" data-page="discover">