fix: use shared (non-exclusive) COM port access on Windows

pyserial opens COM ports with dwShareMode=0 (exclusive) by default.
On Windows 10/11 this can cause PermissionError(13) even when the port
is not visibly occupied — the OS may have an internal handle.

Added _open_port() helper that tries exclusive=False (FILE_SHARE_READ|
FILE_SHARE_WRITE) first on Windows, falling back to exclusive if
pyserial < 3.3. Applied to all 7 serial.Serial() call sites across
serial_monitor, reboot_manager, 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_port
+                ser = _open_port(self._port, self._baudrate, 1)
                 ser.close()
                 self.after(0, self._on_open_success)
             except Exception as e:

src/reboot_manager.py

@@ -125,11 +125,12 @@ def reboot_via_serial(
 
     try:
         import serial
+        from serial_monitor import _open_port
 
-        with serial.Serial(
+        with _open_port(
             config.serial_port,
             config.serial_baudrate,
-            timeout=5,
+            timeout=5.0,
         ) as ser:
             ser.reset_input_buffer()
             # Send Ctrl+C to break out of any running process

src/serial_monitor.py

@@ -7,6 +7,7 @@ Provides UartMonitor for continuous background monitoring.
 
 from __future__ import annotations
 
+import sys
 import re
 import threading
 import time
@@ -17,6 +18,23 @@ import serial
 import serial.tools.list_ports
 
 
+def _open_port(port: str, baudrate: int, timeout: float = 1.0) -> serial.Serial:
+    """Open a serial port with Windows-friendly settings.
+
+    On Windows, tries exclusive=False first (shared access) to avoid
+    PermissionError(13) when the port appears free but CreateFile
+    with dwShareMode=0 returns ERROR_ACCESS_DENIED.
+
+    Falls back to default (exclusive) if pyserial < 3.3.
+    """
+    if sys.platform == "win32":
+        try:
+            return serial.Serial(port, baudrate, timeout=timeout, exclusive=False)
+        except TypeError:
+            pass  # pyserial < 3.3, doesn't support exclusive=
+    return serial.Serial(port, baudrate, timeout=timeout)
+
+
 @dataclass
 class BootInfo:
     """Parsed information extracted from Zynq boot output."""
@@ -169,7 +187,7 @@ def check_uart_available(
 
     # 3. Try to open and read
     try:
-        with serial.Serial(port, baudrate, timeout=timeout) as ser:
+        with _open_port(port, baudrate, timeout) as ser:
             ser.reset_input_buffer()
             lines: list[str] = []
             while ser.in_waiting:
@@ -243,7 +261,7 @@ def test_serial_version(
     import serial
 
     try:
-        with serial.Serial(port, baudrate, timeout=timeout) as ser:
+        with _open_port(port, baudrate, timeout) as ser:
             ser.reset_input_buffer()
             ser.reset_output_buffer()
 
@@ -384,7 +402,7 @@ class UartMonitor:
 
         # Quick port check before starting thread
         try:
-            test_ser = serial.Serial(self._port, self._baudrate, timeout=1)
+            test_ser = _open_port(self._port, self._baudrate, 1)
             test_ser.close()
         except serial.SerialException as e:
             if self.on_error:
@@ -410,7 +428,7 @@ class UartMonitor:
     def _read_loop(self) -> None:
         """Main read loop running in background thread."""
         try:
-            ser = serial.Serial(
+            ser = _open_port(
                 self._port,
                 self._baudrate,
                 timeout=self._timeout,