fix(settings): don't close Settings when clicking the busy overlay

The full-screen #busyOverlay (shown during sync/backup) sits outside
#settingsPanel, so a mousedown on it fired the click-outside handler and
closed Settings mid-sync. Exempted #busyOverlay like .modal/.toast already are.

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

js/app.js

@@ -8602,6 +8602,7 @@ async function init() {
         if (e.target.closest('.modal')) return;
         if (e.target.closest('.cmd-palette')) return;
         if (e.target.closest('.toast')) return;
+        if (e.target.closest('#busyOverlay')) return;  // sync/backup spinner
         closeSettings();
     });