fix(tui): 初始化时自动聚焦 textarea

根因: OpenTUI textarea 即使 focused=true 也不会自动获取键盘焦点，
需要显式调用 textarea.focus()。onMount 后延迟 50ms 聚焦，
确保 textarea 挂载完成后 Enter 键可触发 onsubmit。

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/tui/src/TuiApp.tsx

@@ -254,6 +254,11 @@ function AirCodingView(props: {
   let pasteTick: ReturnType<typeof setTimeout> | undefined
   let exitConfirmUntil = 0
 
+  onMount(() => {
+    // Give the textarea time to mount before focusing
+    setTimeout(() => focusPrompt(), 50)
+  })
+
   props.bindState({
     setProjection,
     setView: setActiveView,