feat: Add email field to profile edit form

- Add email input field to profile edit form
- Display current email or 'No email address set' message
- Show helpful hint when adding email for the first time
- Update backend to accept email in profile updates
- Add email uniqueness validation
- Allow users to add/update/remove email address
- Support empty string to remove email (converts to null)

Users can now add or update their email address from the profile page, enabling them to receive email notifications.

src/services/userService.ts

@@ -31,6 +31,7 @@ export interface UpdateProfileDTO {
   firstName?: string;
   lastName?: string;
   username?: string;
+  email?: string;
 }
 
 export interface NotificationPreferences {