feat(sprints): interface de création et gestion des sprints (frontend)

Ticket #11 — surface UI de gestion des sprints (frontend pur).

- Nouveau composant SprintManager : création (nom optionnel), édition et
  gestion du cycle de vie d'un sprint, contrôles accessibles (pas de DnD).
- useTickets étendu aux opérations de gestion de sprint ; ports, adaptateurs
  (ticket.ts, mock/index.ts) et TicketsPanel câblés en conséquence.
- Tests Vitest associés (tickets.test.tsx).

Typecheck / vitest (497+ tests) / build verts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/tickets/index.ts

@@ -7,6 +7,8 @@ export { TicketsPanel } from "./TicketsPanel";
 export type { TicketsPanelProps } from "./TicketsPanel";
 export { TicketDetail } from "./TicketDetail";
 export type { TicketDetailProps } from "./TicketDetail";
+export { SprintManager } from "./SprintManager";
+export type { SprintManagerProps } from "./SprintManager";
 export { useTickets } from "./useTickets";
 export type { TicketsViewModel } from "./useTickets";
 export { useTicketDetail } from "./useTicketDetail";