fix: avoid serial port conflict between workflow and UART window

Flow execution's _start_uart_monitor() now checks if a UART window
is already monitoring before starting its own UartMonitor instance.
This prevents two UartMonitor threads from fighting over the same
serial port.

Changes:
- UartMonitorWindow: add is_monitoring property
- MainWindow._uart_window: store reference to open UART window
- _start_uart_monitor(): skip if UART window already monitoring
- _on_close(): destroy UART window on main window close

src/gui/widgets.py

@@ -1024,6 +1024,11 @@ class UartMonitorWindow(ctk.CTkToplevel):
 
     # ── Window Close ───────────────────────────────────────────
 
+    @property
+    def is_monitoring(self) -> bool:
+        """True if the UART monitor thread is currently running."""
+        return self._monitor is not None and self._monitor.is_running()
+
     def _on_close(self) -> None:
         """Cleanup UartMonitor and destroy window."""
         if self._monitor: