fix: UART window stability on Windows — lift/focus, remove stray grab_release

CTkToplevel on Windows can fail to display (flash then disappear)
without explicit lift()/focus(). Also removed grab_release() call
in _on_close since grab_set() was never called — calling release
without a matching grab can cause issues on some Windows tkinter
builds.

src/gui/widgets.py

@@ -953,6 +953,8 @@ class UartMonitorWindow(ctk.CTkToplevel):
         self.geometry("680x540")
         self.minsize(480, 360)
         self.protocol("WM_DELETE_WINDOW", self._on_close)
+        self.lift()   # ensure visible on Windows
+        self.focus()
 
         self._on_log = on_log
 
@@ -1201,5 +1203,4 @@ class UartMonitorWindow(ctk.CTkToplevel):
         if self._monitor:
             self._monitor.stop()
             self._monitor = None
-        self.grab_release()
         self.destroy()