build(web): build frontend UI automatically in build.rs

vnt-web/build.rs now runs the pnpm UI build when static/ is missing or
older than ui/src, so the embedded assets no longer need to be committed.
Set VNT_WEB_SKIP_UI_BUILD=1 to skip (a placeholder page is written).
Remove vnt-web/static from version control.

.gitignore

@@ -11,3 +11,6 @@ vnt-desktop/dist
 vnt-desktop/src-tauri/gen
 vnt-desktop/src-tauri/icons/android
 vnt-desktop/src-tauri/icons/ios
+
+# 前端构建产物，由 vnt-web/build.rs 自动构建
+vnt-web/static