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

frontend/src/services/api.js

@@ -495,4 +495,16 @@ export const adminAPI = {
   },
 };
 
+// Public API (no authentication required)
+export const publicAPI = {
+  async log404(path, search) {
+    const data = await fetchAPI('/public/log-404', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ path, search }),
+    });
+    return data;
+  },
+};
+
 export { ApiError };