Add remark-gfm for markdown table rendering in Knowledge Base

Tables are a GitHub Flavored Markdown extension not supported by
react-markdown's default parser. Added remark-gfm plugin so tables,
strikethrough, and task lists render correctly in KB articles.

frontend/src/components/KnowledgeBaseViewer.js

@@ -1,5 +1,6 @@
 import React, { useState, useEffect, useRef } from 'react';
 import ReactMarkdown from 'react-markdown';
+import remarkGfm from 'remark-gfm';
 import rehypeSanitize from 'rehype-sanitize';
 import mermaid from 'mermaid';
 import { X, Download, Loader, AlertCircle, FileText, File } from 'lucide-react';
@@ -234,6 +235,7 @@ export default function KnowledgeBaseViewer({ article, onClose }) {
             {isMarkdown && (
               <div className="markdown-content">
                 <ReactMarkdown
+                  remarkPlugins={[remarkGfm]}
                   rehypePlugins={[rehypeSanitize]}
                   components={{
                     code({ inline, className, children }) {