feat(v2): migrate UI from Walk to Gio

Rewrites internal/ui/window.go in Gio, keeping the Window interface
(NewWindow/Run/Present/PresentAlert/ReportIssue) and the pure-Go ViewModel.
Background updates mutate shared state under a lock and Invalidate; the Gio
loop renders the latest frame. Dual tabs, widget.Image for render's RGBA, a
self-drawn confirm dialog and a MessageBeep sound. Drops lxn/walk (and the
TTM_ADDTOOL/comctl class of problems). Verified via GOOS=windows CGO_ENABLED=0
build of the UI packages; full app build needs Windows+CGo for onnxruntime.

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

v2/internal/ui/sound_windows.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package ui
+
+import "golang.org/x/sys/windows"
+
+// MB_ICONHAND selects the system critical-stop sound for a confirmed fall.
+const mbIconHand = 0x00000010
+
+var (
+	user32          = windows.NewLazySystemDLL("user32.dll")
+	procMessageBeep = user32.NewProc("MessageBeep")
+)
+
+func playAlertSound() {
+	_, _, _ = procMessageBeep.Call(uintptr(mbIconHand))
+}