✨ feat: UART Monitor standalone window

Replace 'Read Serial' button with 'Open UART' that opens a new
UART Monitor window (CTkToplevel) with:

- Title bar with status indicator (● connecting/ready/monitoring/error)
- Port info display + Start/Stop/Clear buttons
- Large scrollable log area (CTkTextbox) with monospace font
- Auto-scroll to bottom, line counter in status bar
- Boot info parsing (IP, version) shown as [BOOT] prefix
- Background serial monitoring via UartMonitor class

UART unavailable handling:
- On init, tries to open serial port in background thread
- If port can't open → shows 'UART 不可用' state with error message
- If port opens → shows 'Ready', user clicks Start to begin monitoring

Window features:
- Modal (grab_set) — focus stays on monitor window
- Clean up UartMonitor on close (WM_DELETE_WINDOW handler)
- Thread-safe UI updates via after(0, ...)
- Reuses existing UartMonitor from serial_monitor.py

src/gui/main_window.py

@@ -53,6 +53,7 @@ from gui.widgets import (
     FileSelector,
     LogDisplay,
     SubStepFrame,
+    UartMonitorWindow,
 )
 
 
@@ -591,14 +592,14 @@ class MainWindow(ctk.CTk):
         )
         refresh_btn.grid(row=0, column=2, padx=PADDING_SMALL)
 
-        # Read button
-        read_btn = ctk.CTkButton(
+        # Open UART button
+        uart_btn = ctk.CTkButton(
             panel,
-            text="Read Serial",
+            text="Open UART",
             font=FONT_BODY,
             command=self._read_serial,
         )
-        read_btn.grid(row=2, column=0, padx=PADDING, pady=PADDING_SMALL)
+        uart_btn.grid(row=2, column=0, padx=PADDING, pady=PADDING_SMALL)
 
         # Test version button
         self._test_version_btn = ctk.CTkButton(
@@ -1202,33 +1203,10 @@ class MainWindow(ctk.CTk):
         self.destroy()
 
     def _read_serial(self) -> None:
-        """Read and display serial output."""
+        """Open the UART Monitor window."""
         port = self._port_var.get()
-        if not port:
-            self._log_message("No serial port selected")
-            return
-
-        self._log_message(f"Reading from {port}...")
-        try:
-            import serial
-
-            with serial.Serial(port, self._config.serial_baudrate if self._config else 115200, timeout=5) as ser:
-                ser.reset_input_buffer()
-                lines = []
-                while ser.in_waiting:
-                    line = ser.readline().decode("utf-8", errors="replace").strip()
-                    if line:
-                        lines.append(line)
-
-            if lines:
-                info = parse_boot_output(lines)
-                self._log_message(f"  Parsed: IP={info.ip_address}, Ver={info.version}")
-                for line in lines[-10:]:
-                    self._log_message(f"  > {line}")
-            else:
-                self._log_message("  No data received")
-        except Exception as e:
-            self._log_message(f"  Error: {e}")
+        baudrate = self._config.serial_baudrate if self._config else 115200
+        UartMonitorWindow(self, port=port, baudrate=baudrate)
 
     def _test_serial_version(self) -> None:
         """Test serial port by sending 'ver()' command and parsing response."""