fix(tui): 交换 Enter 和 Ctrl+Enter 行为

Enter → 提交文本（handleKeyDown 拦截 return 键）
Ctrl+Enter → 换行（handleKeyDown 拦截 ctrl+return，插入 \n）

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

packages/tui/src/TuiApp.tsx

@@ -355,6 +355,18 @@ function AirCodingView(props: {
       return
     }
 
+    // Ctrl+Enter inserts newline at cursor
+    if (event.ctrl && event.name === 'return') {
+      event.preventDefault()
+      if (textarea && !textarea.isDestroyed) {
+        const pos = textarea.cursorOffset
+        const current = textarea.plainText
+        textarea.setText(current.slice(0, pos) + '\n' + current.slice(pos))
+        textarea.cursorOffset = pos + 1
+      }
+      return
+    }
+
     if (event.ctrl && event.name === 'c') {
       event.preventDefault()
       if (textarea && !textarea.isDestroyed && textarea.plainText.length > 0) {