fix: defensive attribute access in _sync_config_from_ui

_sync_config_from_ui() now uses hasattr() for _port_var, _erase_cb_var,
_xilinx_path_var, and _ip_string_var so it doesn't crash when called
during _build_config_panel() before those widgets are created.

This fixes the AttributeError: '_tkinter.tkapp' object has no attribute
'_port_var' that occurred on startup.

src/gui/main_window.py

@@ -218,15 +218,20 @@ class MainWindow(ctk.CTk):
         object's perspective); the FileSelector stores both the
         absolute path (for internal use) and the relative path
         (for saving to YAML).
+
+        All attribute accesses are defensive — widgets may not exist
+        yet during construction (e.g. _port_var created in _build_serial).
         """
         if not self._config:
             return
-        # Sync IP address
+        # Sync IP address (may not exist yet during _build_config_panel)
         if self._ip_string_var:
             self._config.zynq_ip = self._ip_string_var.get()
         # Sync Xilinx Kit path
+        if hasattr(self, '_xilinx_path_var'):
             self._config.xilinx_path = self._xilinx_path_var.get()
-        # Sync serial port
+        # Sync serial port (may not exist yet during _build_config_panel)
+        if hasattr(self, '_port_var'):
             self._config.serial_port = self._port_var.get()
         # Sync file paths from FileSelectors
         files = self._get_selected_files()
@@ -245,6 +250,8 @@ class MainWindow(ctk.CTk):
                 if abs_path:
                     self._config._config_path  # ensure _config_path is set
                     setattr(self._config, attr, self._config._relative_path(abs_path))
+        # Sync erase checkbox (may not exist yet)
+        if hasattr(self, '_erase_cb_var'):
             self._config.erase_all = self._erase_cb_var.get()
 
     def _auto_save_config(self) -> None: