feat(admin): media library + upload component (replace URL fields)

- /admin/files Media Library: drag-drop multi-upload, thumbnails, copy-URL, delete
- FileUploadField replaces raw URL inputs; new "image" field type in AdminResource;
  wired into category image
- upload proxy /api/admin/files/upload: browser → Next → presigned PUT (server-side,
  reaches minio:9000) → confirm → returns public URL
- user-uploads bucket is public-read; public base via NEXT_PUBLIC_MINIO_URL

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/admin/admin-resources.tsx

@@ -51,7 +51,7 @@ export const categoriesConfig: ResourceConfig = {
     { key: "name", label: "Name", required: true },
     { key: "slug", label: "Slug", required: true },
     { key: "description", label: "Description / content", type: "textarea" },
-    { key: "image_url", label: "Image URL" },
+    { key: "image_url", label: "Image", type: "image" },
     { key: "icon", label: "Icon" },
     // SEO
     { key: "meta_title", label: "SEO · Meta title" },