fix: use s.code as location select value, fall back to slugified name

The server webhook will soon return a 'code' field (e.g. 'fr',
'usa') alongside the display 'server' name. Use it as the option
value so the payment webhook receives the correct location code.
Falls back to the old slugified server name when code is absent
(e.g. current live API).

add.html

@@ -564,10 +564,11 @@
             }
             servers.forEach(function(s) {
                 const server = s.server || '';
+                const serverCode = s.code || server.toLowerCase().replace(/\s+/g, '-');
                 const available = parseInt(s['Seats Available'], 10) || 0;
                 const label = server.charAt(0).toUpperCase() + server.slice(1);
                 const opt = document.createElement('option');
-                opt.value = server.toLowerCase().replace(/\s+/g, '-');
+                opt.value = serverCode;
                 if (available <= 0) {
                     opt.textContent = label + ' (sold out)';
                     opt.disabled = true;