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/pages/issue.page.scss

@@ -345,3 +345,39 @@ a.ts-key {
   gap: 0.5rem;
   margin: 0.5rem 0;
 }
+
+.ts-mention {
+  padding: 0 0.2em;
+  border-radius: 0.25em;
+  background: color-mix(in oklab, var(--ts-brand) 14%, transparent);
+  color: inherit;
+  font-weight: 600;
+}
+
+.ts-mention.is-me {
+  background: color-mix(in oklab, var(--ts-brand) 30%, transparent);
+}
+
+.ts-mentions {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.375rem;
+}
+
+.ts-mentions__option {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.25rem 0.625rem 0.25rem 0.25rem;
+  border: 1px solid rgba(127, 127, 127, 0.3);
+  border-radius: 999px;
+  background: transparent;
+  color: inherit;
+  font: inherit;
+  cursor: pointer;
+}
+
+.ts-mentions__option:hover,
+.ts-mentions__option:focus-visible {
+  border-color: currentColor;
+}

libs/web/tracker/presenters/src/lib/pages/issue.page.ts

@@ -35,9 +35,13 @@ import {
 import {
   Attachment,
   Comment,
+  insertMention,
   IssueLink,
   IssueLinkType,
   IssuePriority,
+  matches,
+  mentionQuery,
+  mentionSegments,
   ReleaseStatus,
 } from '@task-stream/web/tracker/domain';
 import {
@@ -95,6 +99,20 @@ const MAX_FILE_BYTES = 25 * 1024 * 1024;
             </div>
             <div class="ts-issue__title-row">
               <h1>{{ issue.title }}</h1>
+              @if (project.myRole()) {
+                <pi-action-button
+                  size="small"
+                  color="grey"
+                  appearance="outline"
+                  [text]="
+                    facade.watching()
+                      ? 'Siguiendo · ' + issue.watchers.length
+                      : 'Seguir'
+                  "
+                  [attr.aria-pressed]="facade.watching()"
+                  (execute)="facade.setWatching(!facade.watching())"
+                />
+              }
               @if (project.canContribute()) {
                 <pi-icon-button
                   icon="edit"
@@ -287,7 +305,19 @@ const MAX_FILE_BYTES = 25 * 1024 * 1024;
                       />
                     }
                   </div>
-                  <p class="ts-prewrap">{{ comment.body }}</p>
+                  <p class="ts-prewrap">
+                    @for (part of segments(comment.body); track $index) {
+                      @if ('mention' in part) {
+                        <span
+                          class="ts-mention"
+                          [class.is-me]="part.userId === facade.me()"
+                          >@{{ part.mention }}</span
+                        >
+                      } @else {
+                        {{ part.text }}
+                      }
+                    }
+                  </p>
                   @if (filesOf(comment).length) {
                     <div class="ts-attachments">
                       @for (file of filesOf(comment); track file.id) {
@@ -308,9 +338,29 @@ const MAX_FILE_BYTES = 25 * 1024 * 1024;
                 novalidate
               >
                 <pi-textarea
-                  placeholder="Deja una decisión, una explicación, un paso para reproducir…"
+                  placeholder="Deja una decisión, una explicación, un paso para reproducir… Escribe @ para mencionar a alguien."
                   [(value)]="commentBody"
                 />
+                @if (mentionOptions().length) {
+                  <div
+                    class="ts-mentions"
+                    role="group"
+                    aria-label="Mencionar a"
+                  >
+                    @for (member of mentionOptions(); track member.id) {
+                      <button
+                        type="button"
+                        class="ts-mentions__option"
+                        (click)="mention(member)"
+                      >
+                        <span class="ts-comment__avatar" aria-hidden="true">{{
+                          initialsOf(member.name)
+                        }}</span>
+                        {{ member.name }}
+                      </button>
+                    }
+                  </div>
+                }
                 <pi-file-upload
                   label="Adjuntar al comentario"
                   [multiple]="true"
@@ -576,6 +626,19 @@ export class IssuePage {
   protected readonly linkTarget = signal('');
   protected readonly commentBody = signal('');
   protected readonly commentFiles = signal<File[]>([]);
+  protected readonly segments = mentionSegments;
+  /** Members matching what is typed after an `@` at the end of the comment. */
+  protected readonly mentionOptions = computed(() => {
+    const query = mentionQuery(this.commentBody());
+    if (query === null) return [];
+    return this.project
+      .members()
+      .map((m) => m.user)
+      .filter(
+        (user) => user.id !== this.facade.me() && matches(user.name, query),
+      )
+      .slice(0, 6);
+  });
   protected readonly pendingFiles = signal<File[]>([]);
   protected readonly maxFileBytes = MAX_FILE_BYTES;
 
@@ -679,6 +742,10 @@ export class IssuePage {
     this.commentFiles.set([]);
   }
 
+  protected mention(user: { id: string; name: string }): void {
+    this.commentBody.update((text) => insertMention(text, user));
+  }
+
   protected attach(): void {
     this.facade.attach(this.pendingFiles());
     this.pendingFiles.set([]);

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;