feat(system): implement 404 page with activity logging and change profile route format

Backend Changes:
- Added public API endpoint /api/public/log-404 (no auth required)
- Created backend/src/routes/public.js for public endpoints
- Added ACTIONS.SYSTEM_404 and CATEGORIES.system to activity log service
- Registered public routes in app.js

Frontend Changes:
- Created NotFoundPage.jsx with standalone layout (no auth required)
- Added publicAPI.log404() to log 404 access attempts
- Logs both authenticated and anonymous users
- Changed profile route from /@:username to /u/:username
- Made profile route public (removed ProtectedRoute wrapper)
- Updated all profile links from /@${username} to /u/${username} in:
  - ChatMessage.jsx
  - DashboardMatchCard.jsx
  - MatchRequestCards.jsx
  - MatchCard.jsx
  - UserListItem.jsx
  - MatchChatPage.jsx
  - PublicProfilePage.jsx

Fixes:
- React Router doesn't support @ in path segments
- 404 page now accessible to non-authenticated users without redirect
- Profile route no longer catches all unmatched routes

backend/src/services/activityLog.js

@@ -34,6 +34,9 @@ const ACTIONS = {
   CHAT_MESSAGE: 'chat.message',
   CHAT_JOIN_ROOM: 'chat.join_room',
   CHAT_LEAVE_ROOM: 'chat.leave_room',
+
+  // System actions
+  SYSTEM_404: 'system.404',
 };
 
 // Category mapping from action
@@ -43,6 +46,7 @@ const CATEGORIES = {
   'match': 'match',
   'admin': 'admin',
   'chat': 'chat',
+  'system': 'system',
 };
 
 /**