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/components/matches/MatchCard.jsx

@@ -16,7 +16,7 @@ const MatchCard = ({ match, onAccept, onReject, onOpenChat, processing }) => {
       <div className="flex items-start justify-between">
         <div className="flex-1">
           <div className="flex items-center gap-3 mb-2">
-            <Link to={`/@${match.partner.username}`} className="flex-shrink-0">
+            <Link to={`/u/${match.partner.username}`} className="flex-shrink-0">
               <img
                 src={match.partner.avatar || `https://api.dicebear.com/7.x/avataaars/svg?seed=${match.partner.username}`}
                 alt={match.partner.username}
@@ -24,7 +24,7 @@ const MatchCard = ({ match, onAccept, onReject, onOpenChat, processing }) => {
               />
             </Link>
             <div>
-              <Link to={`/@${match.partner.username}`}>
+              <Link to={`/u/${match.partner.username}`}>
                 <h3 className="font-semibold text-gray-900 hover:text-primary-600 transition-colors">
                   {match.partner.firstName && match.partner.lastName
                     ? `${match.partner.firstName} ${match.partner.lastName}`