feat: add user preferences dashboard

- Add /preferences route and preferences.html template for user settings
- Implement username, email, password, and theme update functionality
- Add account deletion feature with confirmation
- Add navigation link to preferences from dashboard
- Create API endpoints:
  - PUT /api/user/username - Update username
  - PUT /api/user/email - Update email address
  - PUT /api/user/password - Change password with verification
  - DELETE /api/user/account - Delete user account
- Add database queries for user updates and account deletion
- Create Bruno API testing files for all user preference endpoints
- Add proper validation, error handling, and security checks

bruno/user/Delete Account.bru

@@ -0,0 +1,39 @@
+meta {
+  name: Delete Account
+  type: http
+  seq: 4
+}
+
+delete {
+  url: {{base_url}}/api/user/account
+  body: none
+  auth: inherit
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## Delete Account
+
+  Permanently deletes the authenticated user's account and all associated data.
+
+  **Method:** DELETE
+
+  **Endpoint:** /api/user/account
+
+  **Authentication:** Required
+
+  **Request Body:** None
+
+  **Response:**
+  - `message` (string): Success message
+
+  **Status Codes:**
+  - 200: Success
+  - 401: Unauthorized
+
+  **Warning:** This action cannot be undone and will permanently delete all user data including ebooks, ratings, and progress.
+}