Restrict server detail access for guests

- Add isGuest flag to UserContext
- Block guests from navigating to /server/:id route
- Make ServerCards non-clickable for guests
- Add rejectGuest middleware to backend
- Protect server detail endpoints (/:id, /metrics/history, /whitelist)

Guests can now only view the dashboard overview without accessing
individual server details.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gsm-frontend/src/pages/Dashboard.jsx

@@ -255,8 +255,9 @@ export default function Dashboard({ onLogout }) {
               >
                 <ServerCard
                   server={server}
-                  onClick={() => navigate('/server/' + server.id)}
+                  onClick={isGuest ? undefined : () => navigate('/server/' + server.id)}
                   isAuthenticated={isAuthenticated}
+                  isGuest={isGuest}
                   displaySettings={displaySettings[server.id]}
                 />
               </div>