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

bruno/library/Get Scan Settings.bru

@@ -0,0 +1,36 @@
+meta {
+  name: Get Scan Settings
+  type: http
+  seq: 2
+}
+
+get {
+  url: {{base_url}}/api/library/scan-settings
+  body: none
+  auth: inherit
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## Get Scan Settings
+
+  Retrieves the user's current ebook scanning settings.
+
+  **Method:** GET
+
+  **Endpoint:** /api/library/scan-settings
+
+  **Authentication:** Required
+
+  **Response:**
+  - `scan_frequency_minutes` (integer): Minutes between automatic scans
+  - `auto_scan_enabled` (boolean): Whether automatic scanning is enabled
+
+  **Status Codes:**
+  - 200: Success
+  - 401: Unauthorized
+}

bruno/library/Update Scan Settings.bru

@@ -0,0 +1,47 @@
+meta {
+  name: Update Scan Settings
+  type: http
+  seq: 1
+}
+
+put {
+  url: {{base_url}}/api/library/scan-settings
+  body: json
+  auth: inherit
+}
+
+body {
+  {
+    "scan_frequency_minutes": 60,
+    "auto_scan_enabled": true
+  }
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## Update Scan Settings
+
+  Updates the user's ebook scanning settings.
+
+  **Method:** PUT
+
+  **Endpoint:** /api/library/scan-settings
+
+  **Authentication:** Required
+
+  **Request Body:**
+  - `scan_frequency_minutes` (integer, required): Minutes between automatic scans (15-1440)
+  - `auto_scan_enabled` (boolean, required): Whether automatic scanning is enabled
+
+  **Response:**
+  - `message` (string): Success message
+
+  **Status Codes:**
+  - 200: Success
+  - 400: Invalid settings
+  - 401: Unauthorized
+}