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/App.jsx

@@ -19,6 +19,7 @@ import HistoryPage from './pages/HistoryPage';
 import ProfilePage from './pages/ProfilePage';
 import PublicProfilePage from './pages/PublicProfilePage';
 import ActivityLogsPage from './pages/admin/ActivityLogsPage';
+import NotFoundPage from './pages/NotFoundPage';
 import VerificationBanner from './components/common/VerificationBanner';
 import InstallPWA from './components/pwa/InstallPWA';
 
@@ -210,18 +211,15 @@ function App() {
             }
           />
 
-          {/* Public Profile - must be before home route */}
-          <Route
-            path="/:username"
-            element={
-              <ProtectedRoute>
-                <PublicProfilePage />
-              </ProtectedRoute>
-            }
-          />
 
           {/* Home Page */}
           <Route path="/" element={<HomePage />} />
+
+          {/* Public Profile - /u/username format (no auth required) */}
+          <Route path="/u/:username" element={<PublicProfilePage />} />
+
+          {/* 404 Not Found - Catch all unmatched routes */}
+          <Route path="*" element={<NotFoundPage />} />
         </Routes>
       </AuthProvider>
     </BrowserRouter>