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/main_window.py

@@ -82,6 +82,7 @@ class MainWindow(ctk.CTk):
         self._zynq_jtag_present: bool = False
         self._zynq_jtag_info = None
         self._uart_monitor: UartMonitor | None = None
+        self._uart_window: UartMonitorWindow | None = None
         self._flash_phase: str = ""
 
         # StringVar references for config sync
@@ -1148,6 +1149,12 @@ class MainWindow(ctk.CTk):
 
     def _start_uart_monitor(self) -> None:
         """Start background serial monitoring if UART is available."""
+        # If UART window is already monitoring, don't start a second instance
+        if (self._uart_window
+                and self._uart_window.winfo_exists()
+                and self._uart_window.is_monitoring):
+            self._log_message("  [UART] Already monitoring via UART window")
+            return
         if not self._uart_available or not self._config:
             return
         port = self._config.serial_port
@@ -1191,6 +1198,9 @@ class MainWindow(ctk.CTk):
     def _on_close(self) -> None:
         """Handle window close event: save config and cleanup."""
         self._stop_uart_monitor()
+        if self._uart_window and self._uart_window.winfo_exists():
+            self._uart_window.destroy()
+            self._uart_window = None
         if self._config:
             self._sync_config_from_ui()  # Sync UI values first
             # Save to user config file (config.yaml)
@@ -1206,7 +1216,7 @@ class MainWindow(ctk.CTk):
         """Open the UART Monitor window."""
         port = self._port_var.get()
         baudrate = self._config.serial_baudrate if self._config else 115200
-        UartMonitorWindow(self, port=port, baudrate=baudrate)
+        self._uart_window = UartMonitorWindow(self, port=port, baudrate=baudrate)
 
     def _test_serial_version(self) -> None:
         """Test serial port by sending 'ver()' command and parsing response."""

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: