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>

temp-public-policy.sql

@@ -0,0 +1,16 @@
+-- 임시 공개 접근 정책 (테스트용만 사용!)
+-- 보안상 권장하지 않음 - 운영환경에서는 사용하지 마세요
+
+-- 모든 사용자가 files 버킷에 업로드 가능 (임시)
+CREATE POLICY "Public upload for testing"
+ON storage.objects
+FOR INSERT 
+WITH CHECK (bucket_id = 'files');
+
+-- 모든 사용자가 files 버킷 파일 조회 가능 (임시)
+CREATE POLICY "Public read for testing"
+ON storage.objects
+FOR SELECT 
+USING (bucket_id = 'files');
+
+-- 주의: 이 정책들은 테스트 후 반드시 삭제하고 위의 사용자별 정책으로 교체하세요!