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/app/[locale]/admin/layout.tsx

@@ -22,6 +22,7 @@ export default async function AdminLayout({
     { href: "/admin/fonts", label: t("fonts") },
     { href: "/admin/blogs", label: t("blogs") },
     { href: "/admin/slides", label: t("slides") },
+    { href: "/admin/files", label: t("media") },
     { href: "/admin/ai", label: t("aiContent") },
     { href: "/admin/users", label: t("users") },
     { href: "/admin/plans", label: t("plans") },