refactor(frontend): 重构数据库连接方式

- 移除各前端页面中重复的数据库引擎初始化代码
- 在 global_var.py 中统一初始化和存储数据库引擎
- 更新 setting_page.py 和 main.py 中的数据库连接逻辑
- 优化代码结构，提高可维护性和可扩展性

frontend/setting_page.py

@@ -4,13 +4,12 @@ from sqlmodel import Session, select
 from db import get_engine
 from schema import APIProvider
 import os
-
-engine = get_engine(os.path.join(os.path.dirname(__file__), "..", "workdir"))
+from global_var import sql_engine
 
 def setting_page():
     def get_providers() -> List[List[str]]:
         try:  # 添加异常处理
-            with Session(engine) as session:
+            with Session(sql_engine) as session:
                 providers = session.exec(select(APIProvider)).all()
                 return [
                     [p.id, p.model_id, p.base_url, p.api_key or ""]
@@ -21,7 +20,7 @@ def setting_page():
 
     def add_provider(model_id, base_url, api_key):
         try:
-            with Session(engine) as session:
+            with Session(sql_engine) as session:
                 new_provider = APIProvider(
                     model_id=model_id if model_id else None,
                     base_url=base_url if base_url else None,