build: add remote-only mode lang package stub and replacement logic

add empty-lang-stub.js to stub lang package files in remote-only mode,
add webpack plugin logic to replace lang files with stub to reduce bundle size

common/js/empty-lang-stub.js

@@ -0,0 +1,15 @@
+// ============================================================
+// 空 lang 包占位
+//
+// 在 source === 'remote-only' 模式下, lang/ 目录下的所有 .js 会被
+// webpack.NormalModuleReplacementPlugin 替换为这个 stub.
+//
+// 设计要点:
+//   - 导出 default 对象, 结构与真实 lang 包一致: { lang: {...} }
+//   - lang 字段是空对象, 让 lang() 函数 fallback 路径不报错
+//   - 不导出 named export, 避免 tree-shaking 把整个 stub 干掉
+// ============================================================
+
+export default {
+	lang: {}
+};