feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

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

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

@@ -85,6 +85,7 @@ import { projectTone, PUBLIC_TONE, ROLE } from '../ui/copy';
       }
       <nav class="ts-tabs" aria-label="Secciones del proyecto">
         <a routerLink="board" routerLinkActive="is-active">Tablero</a>
+        <a routerLink="epics" routerLinkActive="is-active">Épicas</a>
         <a routerLink="releases" routerLinkActive="is-active">Releases</a>
         <a routerLink="dashboard" routerLinkActive="is-active">Seguimiento</a>
         <a routerLink="analytics" routerLinkActive="is-active">Estadísticas</a>