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/Update Username.bru

@@ -0,0 +1,46 @@
+meta {
+  name: Update Username
+  type: http
+  seq: 1
+}
+
+put {
+  url: {{base_url}}/api/user/username
+  body: json
+  auth: inherit
+}
+
+body {
+  {
+    "username": "newusername"
+  }
+}
+
+settings {
+  encodeUrl: true
+  timeout: 0
+}
+
+docs {
+  ## Update Username
+
+  Updates the authenticated user's username.
+
+  **Method:** PUT
+
+  **Endpoint:** /api/user/username
+
+  **Authentication:** Required
+
+  **Request Body:**
+  - `username` (string, required): New username (3-50 characters)
+
+  **Response:**
+  - `message` (string): Success message
+
+  **Status Codes:**
+  - 200: Success
+  - 400: Invalid username
+  - 401: Unauthorized
+  - 409: Username already taken
+}