feat: implement user profiles and settings

Add comprehensive user profile system allowing users to showcase
their activity and customize their profiles.

Database Changes:
- Added profile fields to User model: slug, displayName, bio, profilePublic
- Added index on slug field for efficient lookups

API Changes:
- Added GET /users/me endpoint to fetch current user
- Added PUT /users/me endpoint to update user settings
- Added GET /users/profile/:identifier endpoint for public profiles
- Updated UserService with profile methods and statistics
- Modified AuthService to include profile fields in user responses

Frontend Changes:
- Created ProfileComponent to display user profiles with stats
- Created SettingsComponent for profile customization
- Added profile and settings routes
- Updated header dropdown menu with profile links
- Enhanced UserService with profile methods
- Added updateUser method to AuthService

Features:
- Custom profile slugs for clean URLs
- Display names separate from usernames
- User bios (up to 500 characters)
- Public/private profile toggle
- Activity statistics (suggestions, likes, comments, acceptance rate)
- Badge display (Staff, Mod, VIP, Discord Member)
- Beautiful witch-themed styling

Closes #45

apps/frontend/src/app/services/auth.service.ts

@@ -123,6 +123,10 @@ export class AuthService {
     return this.user() !== null;
   }
 
+  updateUser(user: User): void {
+    this.currentUser.set(user);
+  }
+
   isAdmin(): boolean {
     return this.user()?.isAdmin === true;
   }