gui: default load port COM11 to match this bench

Completes the port migration (CLI already defaults to COM11/COM4); the
GUI toolbar field still hardcoded COM1. Supply/meter stay "auto".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

testbench/gui.py

@@ -158,7 +158,7 @@ class TestbenchGUI(tk.Tk):
 
         ttk.Label(bar, text="Load:").pack(side=tk.LEFT)
         self._load_port = ttk.Entry(bar, width=8)
-        self._load_port.insert(0, "COM1")
+        self._load_port.insert(0, "COM11")
         self._load_port.pack(side=tk.LEFT, padx=(2, 4))
         ttk.Label(bar, text="@").pack(side=tk.LEFT)
         self._load_baud = ttk.Entry(bar, width=7)