feat(frontend): per-message markdown rendering

Self-contained, HTML-escaped (XSS-safe) markdown for history messages;
each message rendered independently so a syntax error (e.g. unclosed
code fence) cannot leak into the next message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/ChatPage.tsx

@@ -2,6 +2,7 @@ import axios from 'axios'
 import { io, type Socket } from 'socket.io-client'
 import { useEffect, useMemo, useState } from 'react'
 import { useAuth } from '../auth/auth-context'
+import { renderMarkdown } from '../lib/markdown'
 import { guildMembersCenter, joinGuildCenter } from '../lib/center-auth-client'
 
 type MessageItem = {
@@ -470,7 +471,7 @@ export default function ChatPage() {
                     <span className={`meta-badge ${m.wakeup ? 'on' : ''}`}>wakeup={String(m.wakeup)}</span>
                   ) : null}
                 </div>
-                <div className="text">{m.content}</div>
+                <div className="text md" dangerouslySetInnerHTML={{ __html: renderMarkdown(m.content) }} />
                 {devMode ? (
                   <pre className="meta-raw">
 {JSON.stringify(