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.html

@@ -10,6 +10,13 @@
     <div id="ports"></div>
     <h1>VLAN Configuration</h1>
     <form id="vform" action="/vlan.html">
+      <div>
+        <label for="vlanSelect">VLAN auswählen:</label>
+        <select id="vlanSelect" style="margin: 0 0 0 8px">
+          <option value="" disabled selected>— VLAN wählen —</option>
+        </select>
+      </div>
+      <br/>
       <div>
         <label for="vid">VLAN ID:</label>
         <input type="number" min="1" max="4094" id="vid" name="vid">