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/index.css

@@ -556,6 +556,76 @@ button {
   color: var(--text-faint);
   font-style: italic;
 }
+.md > *:first-child {
+  margin-top: 0;
+}
+.md > *:last-child {
+  margin-bottom: 0;
+}
+.md p {
+  margin: 4px 0;
+}
+.md h1,
+.md h2,
+.md h3,
+.md h4,
+.md h5,
+.md h6 {
+  margin: 8px 0 4px;
+  line-height: 1.25;
+}
+.md h1 {
+  font-size: 20px;
+}
+.md h2 {
+  font-size: 18px;
+}
+.md h3 {
+  font-size: 16px;
+}
+.md ul,
+.md ol {
+  margin: 4px 0;
+  padding-left: 22px;
+}
+.md li {
+  margin: 2px 0;
+}
+.md a {
+  color: var(--accent);
+}
+.md code {
+  font-family: var(--mono);
+  font-size: 12.5px;
+  background: var(--elevated);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  padding: 1px 5px;
+}
+.md pre {
+  margin: 6px 0;
+  padding: 10px 12px;
+  background: var(--elevated);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  overflow: auto;
+}
+.md pre code {
+  border: 0;
+  padding: 0;
+  background: none;
+  font-size: 12.5px;
+  white-space: pre;
+}
+.md blockquote {
+  margin: 6px 0;
+  padding: 2px 12px;
+  border-left: 3px solid var(--border);
+  color: var(--text-muted);
+}
+.md del {
+  color: var(--text-faint);
+}
 .meta-badge {
   font-family: var(--mono);
   font-size: 10px;

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(