fix: try both COM11 and \\.\COM11 formats when opening serial port

pyserial should auto-add \\.\ prefix for COM>=10, but some versions
don't. Added _open_serial() helper that tries the raw port name first,
then falls back to explicit NT namespace prefix on Windows.

Replaces all serial.Serial() calls across serial_monitor, reboot_manager,
and widgets.

src/gui/widgets.py

@@ -1077,7 +1077,8 @@ class UartMonitorWindow(ctk.CTkToplevel):
         def _bg_open() -> None:
             try:
                 import serial
-                ser = serial.Serial(self._port, self._baudrate, timeout=1)
+                from serial_monitor import _open_serial
+                ser = _open_serial(self._port, self._baudrate, 1)
                 ser.close()
                 self.after(0, self._on_open_success)
             except Exception as e: