Fix linking existing VPN clients (WireGuard/service protocols and API errors).

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/base.html

@@ -227,10 +227,16 @@
                 throw new Error('Session expired');
             }
 
-            const data = await res.json();
+            const data = await res.json().catch(() => ({}));
 
             if (!res.ok) {
-                throw new Error(data.error || 'Unknown error');
+                const detail = data.error || data.message || data.detail;
+                const msg = typeof detail === 'string'
+                    ? detail
+                    : (Array.isArray(detail)
+                        ? detail.map(d => d.msg || JSON.stringify(d)).join('; ')
+                        : (detail ? JSON.stringify(detail) : `HTTP ${res.status}`));
+                throw new Error(msg || 'Unknown error');
             }
             return data;
         }