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_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package ui
+
+// playAlertSound is a no-op off Windows; the V2 UI targets Windows and uses the
+// system beep there. This stub keeps the package building for cross-checks.
+func playAlertSound() {}