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[];

libs/web/tracker/application/src/lib/facades/issue.facade.ts

@@ -30,6 +30,8 @@ export class IssueFacade {
   readonly error = this.store.error;
   /** Signed-in user's id (authors can edit their comments). */
   readonly me = this.store.me;
+  /** Whether the signed-in person follows the open ticket. */
+  readonly watching = this.store.watching;
 
   /** Loads the ticket (by id or key, e.g. BE-3) and follows its changes. */
   open(idOrKey: string): void {
@@ -85,6 +87,13 @@ export class IssueFacade {
     );
   }
 
+  /** Follow the ticket (or stop): be told when it moves or gets a comment. */
+  setWatching(watching: boolean): void {
+    this.withIssue((issueId) =>
+      this.dispatch.watchRequested({ issueId, watching }),
+    );
+  }
+
   /** Attaches files to the ticket (or under one of its comments). */
   attach(files: File[], commentId: string | null = null): void {
     if (!files.length) return;

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') ?? [],
     ),