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/serial_monitor.py

@@ -8,6 +8,7 @@ Provides UartMonitor for continuous background monitoring.
 from __future__ import annotations
 
 import re
+import sys
 import threading
 import time
 from dataclasses import dataclass
@@ -17,6 +18,33 @@ import serial
 import serial.tools.list_ports
 
 
+def _open_serial(port: str, baudrate: int, timeout: float = 1.0) -> serial.Serial:
+    """Open a serial port, trying both name formats on Windows.
+
+    pyserial normally adds \\\\.\\COM prefix for COM>=10, but some
+    versions don't. Try raw name first, then explicit NT namespace.
+    """
+    port = port.strip()
+    last_err = None
+
+    # Try 1: port name as-is
+    try:
+        return serial.Serial(port, baudrate, timeout=timeout)
+    except serial.SerialException as e:
+        last_err = e
+
+    # Try 2: Windows NT namespace prefix
+    if sys.platform == "win32":
+        m = re.match(r"^(COM\d+)$", port, re.IGNORECASE)
+        if m:
+            try:
+                return serial.Serial(r"\\.\%s" % port, baudrate, timeout=timeout)
+            except serial.SerialException as e:
+                last_err = e
+
+    raise last_err or OSError(f"Cannot open serial port: {port}")
+
+
 @dataclass
 class BootInfo:
     """Parsed information extracted from Zynq boot output."""
@@ -169,7 +197,7 @@ def check_uart_available(
 
     # 3. Try to open and read
     try:
-        with serial.Serial(port, baudrate, timeout=timeout) as ser:
+        with _open_serial(port, baudrate, timeout) as ser:
             ser.reset_input_buffer()
             lines: list[str] = []
             while ser.in_waiting:
@@ -243,7 +271,7 @@ def test_serial_version(
     import serial
 
     try:
-        with serial.Serial(port, baudrate, timeout=timeout) as ser:
+        with _open_serial(port, baudrate, timeout) as ser:
             ser.reset_input_buffer()
             ser.reset_output_buffer()
 
@@ -384,7 +412,7 @@ class UartMonitor:
 
         # Quick port check before starting thread
         try:
-            test_ser = serial.Serial(self._port, self._baudrate, timeout=1)
+            test_ser = _open_serial(self._port, self._baudrate, 1)
             test_ser.close()
         except serial.SerialException as e:
             if self.on_error:
@@ -410,7 +438,7 @@ class UartMonitor:
     def _read_loop(self) -> None:
         """Main read loop running in background thread."""
         try:
-            ser = serial.Serial(
+            ser = _open_serial(
                 self._port,
                 self._baudrate,
                 timeout=self._timeout,