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/stores/issue.store.ts

@@ -60,6 +60,11 @@ export const IssueStore = signalStore(
   }),
   withComputed(({ issue }, auth = inject(AuthFacade)) => ({
     me: computed(() => auth.user()?.id ?? null),
+    watching: computed(
+      () =>
+        !!auth.user() &&
+        (issue()?.watchers ?? []).some((w) => w.id === auth.user()?.id),
+    ),
     outwardLinks: computed(
       () => issue()?.links.filter((l) => l.direction === 'OUTWARD') ?? [],
     ),