feat: add @ prefix to profile URLs and make usernames clickable

- Updated all profile links to use /@username format
- Made usernames clickable in chat messages
- Added URL parameter sanitization to strip @ when fetching user data
- Ensures consistent profile URL format across the application

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={`/${match.partner.username}`} className="flex-shrink-0">
+            <Link to={`/@${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={`/@${match.partner.username}`}>
                 <h3 className="font-semibold text-gray-900 hover:text-primary-600 transition-colors">
                   {match.partner.firstName && match.partner.lastName
                     ? `${match.partner.firstName} ${match.partner.lastName}`