feat: Day 21-22 - knowledge base frontend completed, fix CORS and file upload issues

- Complete knowledge base list and detail pages
- Complete document upload component
- Fix CORS config (add PUT/DELETE method support)
- Fix file upload issues (disabled state and beforeUpload return value)
- Add detailed debug logs (cleaned up)
- Create Day 21-22 completion summary document

frontend/src/pages/KnowledgePage.tsx

@@ -201,7 +201,7 @@ const KnowledgePage: React.FC = () => {
                 <DocumentUpload
                   kbId={currentKb.id}
                   onUploadSuccess={handleUploadSuccess}
-                  disabled={loading}
+                  disabled={false}
                   maxDocuments={50}
                   currentDocumentCount={documents.length}
                 />