feat: render Markdown in AI chat responses (bold, lists, code)

Added react-markdown to render assistant messages with proper formatting:
- **bold** text renders as bold
- Bullet/numbered lists render correctly
- Inline code renders with monospace background
- User messages stay as plain text

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/chat/ChatPanel.tsx

@@ -1,6 +1,7 @@
 import { useState, useRef, useEffect, useCallback } from 'react'
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { MessageSquare, Send, Loader2, X, Plus, Bot, User } from 'lucide-react'
+import ReactMarkdown from 'react-markdown'
 import {
   sendChatMessage,
   getChatSessions,
@@ -230,7 +231,22 @@ export default function ChatPanel({ open, onClose, contextType, contextId }: Cha
                       : { backgroundColor: 'var(--color-bg-surface-hover)', color: 'var(--color-text-content)' }
                   }
                 >
-                  {msg.content}
+                  {msg.role === 'assistant' ? (
+                    <ReactMarkdown
+                      components={{
+                        p: ({ children }) => <p className="mb-1 last:mb-0">{children}</p>,
+                        strong: ({ children }) => <strong className="font-semibold">{children}</strong>,
+                        ul: ({ children }) => <ul className="list-disc list-inside mb-1 space-y-0.5">{children}</ul>,
+                        ol: ({ children }) => <ol className="list-decimal list-inside mb-1 space-y-0.5">{children}</ol>,
+                        li: ({ children }) => <li>{children}</li>,
+                        code: ({ children }) => <code className="px-1 py-0.5 rounded text-xs font-mono" style={{ backgroundColor: 'var(--color-bg-muted)' }}>{children}</code>,
+                      }}
+                    >
+                      {msg.content}
+                    </ReactMarkdown>
+                  ) : (
+                    msg.content
+                  )}
                 </div>
                 <p className="text-[10px] text-content-muted mt-0.5 px-1">
                   {formatTime(msg.created_at)}