feat: a sprint calendar

A Calendario tab lays the project's sprints out month by month as bands,
with the releases and the tickets due on each day. Owners and admins plan
a sprint by clicking the day it starts, and the whole plan downloads as an
iCalendar file. Tickets can be searched by due-date range.

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

libs/web/tracker/presenters/src/lib/pages/project-shell.page.ts

@@ -87,6 +87,7 @@ import { projectTone, PUBLIC_TONE, ROLE } from '../ui/copy';
         <a routerLink="board" routerLinkActive="is-active">Tablero</a>
         <a routerLink="backlog" routerLinkActive="is-active">Backlog</a>
         <a routerLink="sprints" routerLinkActive="is-active">Sprints</a>
+        <a routerLink="calendar" routerLinkActive="is-active">Calendario</a>
         <a routerLink="epics" routerLinkActive="is-active">Épicas</a>
         <a routerLink="roadmap" routerLinkActive="is-active">Hoja de ruta</a>
         <a routerLink="releases" routerLinkActive="is-active">Releases</a>