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/AdminResource.tsx

@@ -2,10 +2,12 @@
 
 import { useCallback, useEffect, useState, type ReactNode } from "react";
 
+import { FileUploadField } from "@/components/admin/FileUploadField";
+
 export interface FieldDef {
   key: string;
   label: string;
-  type?: "text" | "textarea" | "number" | "checkbox" | "select";
+  type?: "text" | "textarea" | "number" | "checkbox" | "select" | "image" | "file";
   options?: { value: string; label: string }[];
   required?: boolean;
   placeholder?: string;
@@ -215,6 +217,12 @@ export function AdminResource({ config }: { config: ResourceConfig }) {
                       <input type="checkbox" checked={!!form[f.key]} onChange={(e) => setForm({ ...form, [f.key]: e.target.checked })} />
                       {f.label}
                     </label>
+                  ) : f.type === "image" || f.type === "file" ? (
+                    <FileUploadField
+                      value={String(form[f.key] ?? "")}
+                      onChange={(url) => setForm({ ...form, [f.key]: url })}
+                      accept={f.type === "image" ? "image/*" : "*/*"}
+                    />
                   ) : (
                     <input type={f.type === "number" ? "number" : "text"} className={inputCls} placeholder={f.placeholder}
                       value={String(form[f.key] ?? "")}