chore(sweep): add modal-bootstrap surface for the standalone bootstrap dialog

The settings-triggered BootstrapDownloadDialog wasn't captured (only the wizard's
bootstrap page was). Add BootstrapDownloadDialog::hide() + a modal-bootstrap sweep
surface (opens the confirm screen) so the migrated dialog is verifiable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/bootstrap_download_dialog.h

@@ -47,6 +47,9 @@ public:
 
     static bool isOpen() { return s_open; }
 
+    // Close the dialog (used by the UI sweep teardown).
+    static void hide() { s_open = false; s_state = State::Confirm; }
+
     static void render() {
         if (!s_app) return;
         if (!s_open) {