fix: defer grab_set() to avoid 'window not viewable' error

CTkToplevel grab_set() fails when called before the window is
fully realized on screen. Defer to after(50ms) so the window
has time to become viewable.

src/gui/widgets.py

@@ -797,9 +797,11 @@ class UartMonitorWindow(ctk.CTkToplevel):
         self.title("UART Monitor")
         self.geometry("680x540")
         self.minsize(480, 360)
-        self.grab_set()
         self.protocol("WM_DELETE_WINDOW", self._on_close)
 
+        # Defer grab_set until window is viewable (avoids "window not viewable" error)
+        self.after(50, self._do_grab)
+
         self._port = port
         self._baudrate = baudrate
         self._monitor: UartMonitor | None = None
@@ -814,6 +816,13 @@ class UartMonitorWindow(ctk.CTkToplevel):
         # Try to open serial port after window is drawn
         self.after(100, self._try_open)
 
+    def _do_grab(self) -> None:
+        """Acquire grab once window is viewable."""
+        try:
+            self.grab_set()
+        except Exception:
+            pass  # Window may have been destroyed already
+
     # ── Title Bar ──────────────────────────────────────────────
 
     def _build_title_bar(self) -> None: