refactor(frontend): colocate admin components and split service layer

Move route-specific admin components from components/common/admin into
app/(main)/admin/<feature>/components/. Split AdminService god object into
domain services, fix UploadService to use BaseService, add AdminUploadService,
consolidate DB export into DbManageService, and migrate consumers to the
unified services entry.

frontend/contexts/admin-context.tsx

@@ -5,7 +5,7 @@ import {createContext, useCallback, useContext, useRef, useState} from "react"
 import {useQueryClient} from "@tanstack/react-query"
 
 import type {SystemConfig, UpdateSystemConfigRequest} from "@/lib/services/admin"
-import {AdminService} from "@/lib/services/admin"
+import services from "@/lib/services"
 import {handleContextError} from "@/lib/utils/error-handling"
 
 
@@ -49,7 +49,7 @@ export function AdminProvider({ children }: { children: React.ReactNode }) {
     try {
       setSystemConfigsLoading(true)
       setSystemConfigsError(null)
-      const data = await AdminService.listSystemConfigs(type)
+      const data = await services.adminSystemConfig.listSystemConfigs(type)
 
       if (requestId !== systemRequestIdRef.current) {
         return
@@ -71,7 +71,7 @@ export function AdminProvider({ children }: { children: React.ReactNode }) {
   /** 更新系统配置 */
   const updateSystemConfig = useCallback(async (key: string, data: UpdateSystemConfigRequest) => {
     try {
-      await AdminService.updateSystemConfig(key, data)
+      await services.adminSystemConfig.updateSystemConfig(key, data)
       await queryClient.invalidateQueries({ queryKey: ['public-config'] })
       await refetchSystemConfigs(lastConfigTypeRef.current)
     } catch (error) {