Resolve MINTR-48 "Feature/"

• Replace mock documents with real API calls using documentService
• Implement getDocumentsByProject to fetch documents on component mount
• Add uploadDocument with category selection, loading state, and error handling
• Add deleteDocument with optimistic UI update and spinner feedback
• Implement download functionality using blob fetch from documentUrl
• Display delete error message and loading states in document list
• Improve document preview modal with larger size (max-w-2xl)