refactor: 抽取 UI 组件、改进错误页面、统一导入路径并简化数据库接口

src/server/db/index.ts

@@ -13,11 +13,7 @@ export type DB = ReturnType<typeof createDB>
 export const getDB = (() => {
   let db: DB | null = null
 
-  return (singleton = true): DB => {
-    if (!singleton) {
-      return createDB()
-    }
-
+  return (): DB => {
     db ??= createDB()
     return db
   }