feat: sprints with burndown and carry-over

Plan sprints with dates and a goal, run one at a time per project, and
put tickets in them from the ticket page. The Sprints tab follows the one
under way with its progress and a burndown in tickets or points against
the ideal line. Closing a sprint sends what was left to the next one or
back to the backlog; the board can be filtered by sprint.

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

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

@@ -86,6 +86,7 @@ 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="backlog" routerLinkActive="is-active">Backlog</a>
+        <a routerLink="sprints" routerLinkActive="is-active">Sprints</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>