feat: create admin dashboard with user preferences and library settings

- Add admin dashboard at /admin route consolidating all user settings
- Move user preferences (username, email, password, theme) from separate page
- Add ebook library preferences section with folder management
- Add scan settings (frequency and auto-scan toggle) with database persistence
- Create database migration for scan_frequency_minutes and auto_scan_enabled columns
- Add API endpoints for scan settings management:
  - PUT /api/library/scan-settings - Update scan preferences
  - GET /api/library/scan-settings - Get current scan settings
- Update dashboard navigation to link to admin dashboard
- Remove old preferences.html template (functionality moved to admin)
- Create Bruno API testing files for library endpoints
- Add real-time folder loading and management in admin interface
- Implement scan settings persistence and retrieval from database

templates/dashboard.html

@@ -12,7 +12,7 @@
             </div>
             <div class="flex items-center space-x-4">
                 <a href="/" class="px-3 py-2 text-sm hover:opacity-80" style="color: var(--text-secondary)">Dashboard</a>
-                <a href="/preferences" class="px-3 py-2 text-sm hover:opacity-80" style="color: var(--text-secondary)">Preferences</a>
+                <a href="/admin" class="px-3 py-2 text-sm hover:opacity-80" style="color: var(--text-secondary)">Admin</a>
                 <span style="color: var(--text-secondary)">Welcome, {{.User.Username}}!</span>
                 <button onclick="logout()" class="btn-primary px-4 py-2 rounded text-sm">
                     Logout