fix: textual.suggestion 兼容老版 Textual + 升级最低版本约束

- textual.suggestion 在 Textual < 2.0 中不存在
- 添加 try/except fallback 构造兼容 Suggestion 类
- requirements.txt: textual>=0.40.0 → >=0.47.0

Co-Authored-By: Claude <noreply@anthropic.com>

services/tui_service.py

@@ -10,7 +10,15 @@ from textual.containers import Container, ScrollableContainer, Horizontal
 from textual.widgets import Static, Input, Header, Footer, LoadingIndicator
 from textual.reactive import reactive
 from textual.suggester import Suggester
-from textual.suggestion import Suggestion
+
+try:
+    from textual.suggestion import Suggestion
+except ImportError:
+    # Textual < 2.0 兼容 — 自行构造 Suggestion
+    class Suggestion:
+        __slots__ = ("value",)
+        def __init__(self, value: str = ""):
+            self.value = value
 from typing import List, Dict
 import asyncio
 from datetime import datetime