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;