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/app.js

@@ -131,6 +131,9 @@ app.get('/api/debug/ip', (req, res) => {
 // Apply rate limiting to all API routes
 app.use('/api/', apiLimiter);
 
+// Public routes (no authentication required)
+app.use('/api/public', require('./routes/public'));
+
 // API routes
 app.use('/api/auth', require('./routes/auth'));
 app.use('/api/users', require('./routes/users'));

backend/src/routes/public.js

@@ -0,0 +1,49 @@
+const express = require('express');
+const { ACTIONS, log: activityLog } = require('../services/activityLog');
+const { getClientIP } = require('../utils/request');
+
+const router = express.Router();
+
+/**
+ * POST /api/public/log-404
+ * Log 404 page access (no authentication required)
+ * Logs both authenticated and unauthenticated users
+ */
+router.post('/log-404', async (req, res) => {
+  try {
+    const { path, search } = req.body;
+
+    // Get user info if authenticated (optional)
+    const userId = req.user?.id || null;
+    const username = req.user?.username || 'anonymous';
+
+    // Construct full path
+    const fullPath = search ? `${path}${search}` : path;
+
+    // Log to activity logs
+    activityLog({
+      userId,
+      username,
+      ipAddress: getClientIP(req),
+      action: ACTIONS.SYSTEM_404,
+      resource: fullPath,
+      method: 'GET',
+      path: fullPath,
+      metadata: {
+        requestedPath: path,
+        queryString: search || null,
+        userAgent: req.headers['user-agent'] || null,
+        referer: req.headers.referer || null,
+      },
+      success: true,
+    });
+
+    res.json({ success: true, logged: true });
+  } catch (error) {
+    console.error('Error logging 404:', error);
+    // Don't fail the request if logging fails
+    res.json({ success: true, logged: false });
+  }
+});
+
+module.exports = router;

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',
 };
 
 /**