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/lib/services/admin/system-config.service.ts

@@ -0,0 +1,44 @@
+import {BaseService} from '@/lib/services/core';
+import type {
+  CreateSystemConfigRequest,
+  SystemConfig,
+  UpdateSystemConfigRequest,
+} from './types';
+
+export class AdminSystemConfigService extends BaseService {
+  protected static readonly basePath = '/api/v1/admin';
+
+  static async createSystemConfig(request: CreateSystemConfigRequest): Promise<void> {
+    return this.post<void>('/system-configs', request);
+  }
+
+  static async listSystemConfigs(type?: 'system' | 'business'): Promise<SystemConfig[]> {
+    const query = type ? `?type=${type}` : '';
+    return this.get<SystemConfig[]>(`/system-configs${query}`);
+  }
+
+  static async getSystemConfig(key: string): Promise<SystemConfig> {
+    return this.get<SystemConfig>(`/system-configs/${key}`);
+  }
+
+  static async updateSystemConfig(key: string, request: UpdateSystemConfigRequest): Promise<void> {
+    return this.put<void>(`/system-configs/${key}`, request);
+  }
+
+  static async testSMTP(request: {
+    smtp_host: string;
+    smtp_port: number;
+    smtp_username: string;
+    smtp_password: string;
+    to: string;
+  }): Promise<{ success: boolean; log: string; error: string }> {
+    return this.post<{ success: boolean; log: string; error: string }>(
+      '/system-configs/smtp/test',
+      request,
+    );
+  }
+
+  static async listUploadTypes(): Promise<string[]> {
+    return this.get<string[]>('/uploads/types');
+  }
+}