feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/presenters/src/lib/ui/notifications-bell.ts

@@ -190,7 +190,7 @@ const ICON: Record<NotificationKind, string> = {
       height: 0.5rem;
       margin-top: 0.5rem;
       border-radius: 50%;
-      background: #a347e1;
+      background: var(--ts-brand, #a347e1);
     }
     .ts-inbox__empty {
       opacity: 0.7;