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/application/src/lib/events/issue.events.ts

@@ -35,6 +35,7 @@ export const issuePageEvents = eventGroup({
       /** Uploaded under the new comment once it exists. */
       files?: File[];
     }>(),
+    watchRequested: type<{ issueId: string; watching: boolean }>(),
     attachRequested: type<{
       issueId: string;
       files: File[];