feat(web): mention people with @ when replying to a comment

The reply box suggests the project's members as the main comment box
does.

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

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

@@ -608,9 +608,33 @@ const MAX_FILE_BYTES = 25 * 1024 * 1024;
                     novalidate
                   >
                     <pi-textarea
-                      [placeholder]="'Responde a ' + thread.comment.author.name + '…'"
+                      [placeholder]="
+                        'Responde a ' +
+                        thread.comment.author.name +
+                        '… Escribe @ para mencionar a alguien.'
+                      "
                       [(value)]="replyBody"
                     />
+                    @if (replyMentionOptions().length) {
+                      <div
+                        class="ts-mentions"
+                        role="group"
+                        aria-label="Mencionar a"
+                      >
+                        @for (member of replyMentionOptions(); track member.id) {
+                          <button
+                            type="button"
+                            class="ts-mentions__option"
+                            (click)="mentionInReply(member)"
+                          >
+                            <span class="ts-comment__avatar" aria-hidden="true">{{
+                              initialsOf(member.name)
+                            }}</span>
+                            {{ member.name }}
+                          </button>
+                        }
+                      </div>
+                    }
                     <div class="ts-reply__actions">
                       <pi-action-button
                         color="primary"
@@ -1024,17 +1048,13 @@ export class IssuePage {
   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 mentionOptions = computed(() =>
+    this.membersMatching(this.commentBody()),
+  );
+  /** The same, for the answer being written. */
+  protected readonly replyMentionOptions = computed(() =>
+    this.membersMatching(this.replyBody()),
+  );
   /** Pull requests first, then the newest. */
   protected readonly devLinks = computed(() =>
     [...(this.facade.issue()?.devLinks ?? [])].sort(
@@ -1226,6 +1246,23 @@ export class IssuePage {
     this.replyingTo.set(null);
   }
 
+  /** Members whose name fits the `@…` being typed at the end of `text`. */
+  private membersMatching(text: string) {
+    const query = mentionQuery(text);
+    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 mentionInReply(user: { id: string; name: string }): void {
+    this.replyBody.update((text) => insertMention(text, user));
+  }
+
   protected mention(user: { id: string; name: string }): void {
     this.commentBody.update((text) => insertMention(text, user));
   }