Add comprehensive file management system with admin/user separation

Features added:
- Admin interface with full CRUD operations and multi-file upload
- User interface with read-only access and download functionality
- Board-style table layout with pagination (10 items per page)
- Category-specific file icons and attachment management
- Drag & drop file upload with preview and individual file removal
- Individual and bulk download with ZIP compression support
- Offline mode with localStorage fallback for both interfaces
- Responsive design with modern UI components

Technical improvements:
- Separated admin (/admin/) and user (/) interfaces
- Enhanced file data structure with consistent naming
- Improved error handling and user notifications
- Multi-file upload processing with base64 encoding
- File type detection and appropriate icon mapping
- Download functionality with single/multiple file handling

🤖 Generated with [Claude Code](https://claude.ai/code)

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

storage-policies.sql

@@ -0,0 +1,39 @@
+-- Storage 전용 정책 (Supabase Dashboard → Storage → Policies에서 실행)
+
+-- 1. 파일 업로드 정책
+CREATE POLICY "Users can upload to own folder"
+ON storage.objects
+FOR INSERT 
+WITH CHECK (
+    bucket_id = 'files' AND 
+    auth.uid()::text = (storage.foldername(name))[1]
+);
+
+-- 2. 파일 조회 정책
+CREATE POLICY "Users can view own files"
+ON storage.objects
+FOR SELECT 
+USING (
+    bucket_id = 'files' AND 
+    auth.uid()::text = (storage.foldername(name))[1]
+);
+
+-- 3. 파일 업데이트 정책
+CREATE POLICY "Users can update own files"
+ON storage.objects
+FOR UPDATE 
+USING (
+    bucket_id = 'files' AND 
+    auth.uid()::text = (storage.foldername(name))[1]
+);
+
+-- 4. 파일 삭제 정책
+CREATE POLICY "Users can delete own files"
+ON storage.objects
+FOR DELETE 
+USING (
+    bucket_id = 'files' AND 
+    auth.uid()::text = (storage.foldername(name))[1]
+);
+
+-- 참고: storage.foldername(name)[1]은 'user_id/filename.txt'에서 'user_id' 부분을 추출합니다.