fix(webui): 修复首次打开慢时多次点击导致的竞态报错

open_kimi_web_embedded 加 launch_lock 串行化并发调用，拿锁后重查单例；
oneshot 在锁内等待主线程建窗完成，封死命令返回后的残留空档。
spawned_by_me 门控失败路径的 server 清理，复用的 server 不再被误杀；
spawn 前用 try_wait 检查已有子进程避免覆盖泄漏。前端两个打开按钮加
busy 态防重入（打开中...）。

src/i18n/en.ts

@@ -528,6 +528,7 @@ export const enTranslations: Record<TranslationKey, string> = {
     "Open the new Web UI in an independent in-app window, or in your system browser (built into kimi-code 0.33+). Requires the kimi CLI on PATH.",
   openWebUIEmbedded: "Open in App",
   openWebUIBrowser: "Open in Browser",
+  webuiOpening: "Opening...",
 
   // Plugin marketplace
   pluginMarketplace: "Plugin Marketplace",