feat(kb): render Mermaid diagrams in Knowledge Base viewer

Installs mermaid v11 and adds a custom ReactMarkdown code renderer
that intercepts fenced mermaid blocks and renders them as SVG diagrams
using the dark theme. SVGs are made responsive (width: 100%).
Non-mermaid code blocks are unchanged.

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

frontend/src/components/KnowledgeBaseViewer.js

@@ -1,7 +1,72 @@
-import React, { useState, useEffect } from 'react';
+import React, { useState, useEffect, useRef } from 'react';
 import ReactMarkdown from 'react-markdown';
+import mermaid from 'mermaid';
 import { X, Download, Loader, AlertCircle, FileText, File } from 'lucide-react';
 
+mermaid.initialize({
+  startOnLoad: false,
+  theme: 'dark',
+  darkMode: true,
+  themeVariables: {
+    background: '#0f172a',
+    primaryColor: '#1e3a5f',
+    primaryTextColor: '#e2e8f0',
+    primaryBorderColor: '#0ea5e9',
+    lineColor: '#475569',
+    secondaryColor: '#1a2e1a',
+    tertiaryColor: '#2d1f14',
+    edgeLabelBackground: '#1e293b',
+    clusterBkg: '#1e293b',
+    titleColor: '#e2e8f0',
+    fontFamily: 'monospace'
+  }
+});
+
+let mermaidCounter = 0;
+
+function MermaidDiagram({ code }) {
+  const ref = useRef(null);
+  const [svgError, setSvgError] = useState('');
+
+  useEffect(() => {
+    let cancelled = false;
+    const id = `mermaid-kb-${++mermaidCounter}`;
+    mermaid.render(id, code)
+      .then(({ svg }) => {
+        if (!cancelled && ref.current) {
+          ref.current.innerHTML = svg;
+          // Make SVG responsive
+          const svgEl = ref.current.querySelector('svg');
+          if (svgEl) {
+            svgEl.removeAttribute('width');
+            svgEl.removeAttribute('height');
+            svgEl.style.width = '100%';
+            svgEl.style.maxWidth = '100%';
+          }
+        }
+      })
+      .catch((err) => {
+        if (!cancelled) setSvgError(err.message || 'Failed to render diagram');
+      });
+    return () => { cancelled = true; };
+  }, [code]);
+
+  if (svgError) {
+    return (
+      <pre style={{ color: '#EF4444', fontSize: '0.75rem', padding: '0.75rem', background: 'rgba(239,68,68,0.1)', borderRadius: '0.375rem', overflowX: 'auto' }}>
+        Mermaid render error: {svgError}
+      </pre>
+    );
+  }
+
+  return (
+    <div
+      ref={ref}
+      style={{ background: 'rgba(15,23,42,0.6)', border: '1px solid rgba(14,165,233,0.2)', borderRadius: '0.5rem', padding: '1rem', margin: '1rem 0', overflowX: 'auto' }}
+    />
+  );
+}
+
 const API_BASE = process.env.REACT_APP_API_BASE || 'http://localhost:3001/api';
 
 export default function KnowledgeBaseViewer({ article, onClose }) {
@@ -167,7 +232,26 @@ export default function KnowledgeBaseViewer({ article, onClose }) {
             {/* Markdown Rendering */}
             {isMarkdown && (
               <div className="markdown-content">
-                <ReactMarkdown>{content}</ReactMarkdown>
+                <ReactMarkdown
+                  components={{
+                    code({ inline, className, children }) {
+                      const lang = /language-(\w+)/.exec(className || '')?.[1];
+                      if (!inline && lang === 'mermaid') {
+                        return <MermaidDiagram code={String(children).replace(/\n$/, '')} />;
+                      }
+                      return (
+                        <code
+                          className={className}
+                          style={inline ? { background: 'rgba(14,165,233,0.15)', padding: '0.1rem 0.3rem', borderRadius: '0.25rem', fontFamily: 'monospace', fontSize: '0.85em' } : {}}
+                        >
+                          {children}
+                        </code>
+                      );
+                    }
+                  }}
+                >
+                  {content}
+                </ReactMarkdown>
               </div>
             )}