ready screen: hide READY! text after first drive, reset on ignition cycle

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/ui/qt/ready.h

@@ -18,6 +18,7 @@ class ReadyWindow : public QWidget {
     Q_OBJECT
 public:
     ReadyWindow(QWidget* parent = nullptr);
+    bool has_driven = false;
 private:
     void showEvent(QShowEvent *event) override;
     void hideEvent(QHideEvent *event) override;
@@ -29,6 +30,7 @@ private:
     QString cur_temp;
     QString error_msg;  // non-empty = show red error instead of READY!
     QElapsedTimer uptime;
+    bool last_started = false;
     QPixmap img_bg;
     QPixmap img_hot;
 };