feat(settings): keep backup pwd on disable, dedicated pwd button, tab icons

Auto-backup no longer wipes the stored password when disabled, so
re-enabling reuses it silently. A dedicated "Set/Change backup password"
button (mirrors sync) owns the password, with a warning status when
unset. Corrected the stale hint that claimed the backup pwd was derived
from the master password. Added icons to each settings tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -8994,6 +8994,7 @@ async function init() {
         e.target.value = n;
         Bridge.setPref(ABK.keep, String(n));
     });
+    $('#autoBackupSetPwdBtn').addEventListener('click', changeBackupPwd);
     $('#autoBackupNowBtn').addEventListener('click', () => runAutoBackupNow());
 
     $('#settingFavicons').addEventListener('change', e => {