fix(apk): Screen lock explique pourquoi il est indisponible au lieu de rester muet

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android-client/web/app.html

@@ -1659,7 +1659,7 @@ function syncUnlockUi() {
     var id = { device: 'sModeDevice', code: 'sModeCode', password: 'sModePwd' }[m];
     document.getElementById(id).classList.toggle('main', unlockMode === m);
   });
-  document.getElementById('sModeDevice').disabled = !canDeviceLock;
+  document.getElementById('sModeDevice').style.opacity = canDeviceLock ? 1 : .4;
   document.getElementById('sModeNote').textContent =
     { device: 'Now: screen lock. Changing the screen lock or adding a fingerprint never deletes the vault.',
       code: 'Now: app code. To go back to the screen lock, type the current code above.',
@@ -1685,6 +1685,7 @@ document.getElementById('sModePwd').addEventListener('click', function () {
   send('pinset?pw=1&c=' + encodeURIComponent(v));
 });
 document.getElementById('sModeDevice').addEventListener('click', function () {
+  if (!canDeviceLock) { toast('No screen lock on this phone - set one in Android settings first'); return; }
   if (unlockMode === 'device') { toast('Already using the screen lock'); return; }
   var v = modeField();
   if (!v && unlockMode !== 'none') { toast('Type the current code or password above first'); return; }