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/state.js

@@ -48,7 +48,7 @@ export const S = {
 
   // Audiobookshelf
   abs: {
-    url: '', user: '', token: null, userId: null,
+    url: '', token: null, userId: null,
     currentLibraryId: null,
   },
   absPlayer: {