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/dashboard/DashboardMatchCard.jsx

@@ -57,7 +57,7 @@ const DashboardMatchCard = ({ match }) => {
     <div className="bg-white rounded-lg shadow-sm border border-gray-200 p-4 hover:shadow-md transition-shadow">
       <div className="flex items-start gap-4">
         {/* Avatar */}
-        <Link to={`/${partner.username}`} className="flex-shrink-0 relative">
+        <Link to={`/@${partner.username}`} className="flex-shrink-0 relative">
           <img
             src={partner.avatar || `https://api.dicebear.com/7.x/avataaars/svg?seed=${partner.username}`}
             alt={partner.username}
@@ -74,7 +74,7 @@ const DashboardMatchCard = ({ match }) => {
         <div className="flex-1 min-w-0">
           <div className="flex items-start justify-between">
             <div>
-              <Link to={`/${partner.username}`}>
+              <Link to={`/@${partner.username}`}>
                 <h3 className="font-semibold text-gray-900 hover:text-primary-600 transition-colors">
                   {partner.firstName && partner.lastName
                     ? `${partner.firstName} ${partner.lastName}`