chore(v2): vendor dependencies for offline/China builds

go mod vendor pins onnxruntime_go v1.12.1, Gio and the rest into v2/vendor
so go run/build work without hitting proxy.golang.org (blocked/slow in
China). Verified: CGO_ENABLED=1 go build -mod=vendor ./internal/spike and
GOOS=windows go build -mod=vendor ./internal/ui both pass.

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

v2/vendor/gioui.org/app/datadir.go

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+//go:build !android
+
+package app
+
+import "os"
+
+func dataDir() (string, error) {
+	return os.UserConfigDir()
+}