fix: auto-start UART monitoring on port open success

_on_open_success(): directly call _on_start() instead of just
enabling the button. User no longer needs to click Start —
monitoring begins automatically when the port is available.

Start/Stop buttons remain for manual control.

src/gui/widgets.py

@@ -928,11 +928,11 @@ class UartMonitorWindow(ctk.CTkToplevel):
         threading.Thread(target=_bg_open, daemon=True).start()
 
     def _on_open_success(self) -> None:
-        """Serial port opened — enable Start button."""
+        """Serial port opened — auto-start monitoring."""
         self._is_opening = False
         self._start_btn.configure(state="normal")
-        self._update_status(ACCENT_PENDING, "Ready")
-        self._log(f"[{self._port}] Port ready. Click Start to begin monitoring.\n")
+        self._log(f"[{self._port}] Port ready, auto-starting monitor...\n")
+        self._on_start()
 
     def _show_unavailable(self, reason: str) -> None:
         """Show UART unavailable state."""