Add confirmation dialog before deleting source documents

Prevents accidental deletion of knowledge base source documents by
prompting the user to confirm before proceeding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/components/KnowledgeBase.tsx

@@ -184,6 +184,7 @@ export const KnowledgeBase: React.FC = () => {
 
     const handleRemoveDoc = async (docId: string) => {
         if (!selectedKb) return;
+        if (!confirm('Are you sure you want to delete this document? This action cannot be undone.')) return;
         try {
             await apiService.removeSourceDocument(selectedKb.id, docId);
             await loadKbDetail(selectedKb.id);