feat(frontend): render <@id> as @name mention chips

Markdown renderer turns <@userId> into a Discord-style mention chip
(@displayName, resolved via guild members; short-id fallback). Tokens
inside backticks stay literal; untranslated <@user.name:..> left as-is.

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

src/index.css

@@ -626,6 +626,17 @@ button {
 .md del {
   color: var(--text-faint);
 }
+.mention {
+  color: #c4b5fd;
+  background: var(--accent-soft);
+  border-radius: 4px;
+  padding: 0 3px;
+  font-weight: 600;
+  white-space: nowrap;
+}
+.mention:hover {
+  background: rgba(168, 85, 247, 0.28);
+}
 .meta-badge {
   font-family: var(--mono);
   font-size: 10px;

src/pages/ChatPage.tsx

@@ -336,6 +336,10 @@ export default function ChatPage() {
   const nameById = new Map(members.map((m) => [m.userId, m.name || m.email]))
   const authorLabel = (uid?: string) =>
     uid ? (uid === session?.user.id ? session?.user.name || 'You' : nameById.get(uid) || uid.slice(0, 8)) : 'unknown'
+  const mentionName = (id: string) =>
+    id === session?.user.id
+      ? session?.user.name || session?.user.email || 'you'
+      : nameById.get(id) || id.slice(0, 8)
 
   const guildById = new Map(members.map((m) => [m.userId, m]))
   const channelMembers = channelMemberIds.map(
@@ -471,7 +475,10 @@ export default function ChatPage() {
                     <span className={`meta-badge ${m.wakeup ? 'on' : ''}`}>wakeup={String(m.wakeup)}</span>
                   ) : null}
                 </div>
-                <div className="text md" dangerouslySetInnerHTML={{ __html: renderMarkdown(m.content) }} />
+                <div
+                  className="text md"
+                  dangerouslySetInnerHTML={{ __html: renderMarkdown(m.content, { resolveMention: mentionName }) }}
+                />
                 {devMode ? (
                   <pre className="meta-raw">
 {JSON.stringify(