Add VLAN selector dropdown to VLAN configuration page

A new <select> element above the VLAN ID input lets users pick an
existing VLAN by name instead of typing the ID. Options are loaded
from /vlanlist on page load and re-loaded after successful
Update/Create operations.

Selection triggers the existing fetchVLAN() flow.

Falls back gracefully if /vlanlist returns an empty list or fails:
the dropdown is hidden and the existing ID input remains functional.

html/vlan_sub.js

@@ -28,6 +28,7 @@ async function vlanSub() {
       });
       console.log('Completed!', response);
     }
+    loadVlanList();
   } catch(err) {
     console.error(`Error: ${err}`);
   }