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/upload/utils.ts

@@ -0,0 +1,18 @@
+export type ImageQuality = 'low' | 'medium' | 'high' | 'origin';
+
+export function getFileUrl(
+  id: string | number | null | undefined,
+  quality: ImageQuality = 'origin',
+): string | null {
+  if (!id) return null;
+  if (quality === 'origin') return `/f/${id}`;
+  return `/f/${id}?quality=${quality}`;
+}
+
+export function formatFileSize(bytes: number): string {
+  if (bytes === 0) return '0 B';
+  const k = 1024;
+  const sizes = ['B', 'KB', 'MB', 'GB'];
+  const i = Math.floor(Math.log(bytes) / Math.log(k));
+  return `${parseFloat((bytes / Math.pow(k, i)).toFixed(1))} ${sizes[i]}`;
+}