feat: add Sync Time button to Device page in web UI

Adds a 'Sync Time' button in the Device Actions section that calls
POST /api/action/sync-time to trigger NTP sync and RTC write.

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

data/app.js

@@ -237,6 +237,7 @@ async function renderDevice(el) {
         <div class="card">
             <h2>Device Actions</h2>
             <div class="btn-group">
+                <button class="btn" id="syncTime">Sync Time</button>
                 <button class="btn btn-danger" onclick="if(confirm('Enter deep sleep?'))api('/api/action/sleep',{method:'POST'})">Deep Sleep</button>
                 <button class="btn btn-danger" onclick="if(confirm('Reboot device?'))fetch('/api/action/reboot',{method:'POST'})">Reboot</button>
             </div>
@@ -263,6 +264,12 @@ async function renderDevice(el) {
             status.style.color = '#c62828';
         }
     };
+
+    document.getElementById('syncTime').onclick = async () => {
+        toast('Syncing...');
+        const result = await api('/api/action/sync-time', { method: 'POST' });
+        toast(result.msg || 'Time synced');
+    };
 }
 
 async function renderFirmware(el) {