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/ports/index.ts

@@ -807,6 +807,28 @@ export interface TicketGateway {
     sprintId: string | null,
     expectedVersion: number,
   ): Promise<Ticket>;
+  /** Creates a sprint (name required by the backend) and returns it (#11). */
+  createSprint(projectId: string, name: string): Promise<Sprint>;
+  /**
+   * Renames a sprint. Optimistic concurrency: `expectedVersion` must match the
+   * sprint's current version (#11).
+   */
+  renameSprint(
+    projectId: string,
+    sprintId: string,
+    name: string,
+    expectedVersion: number,
+  ): Promise<Sprint>;
+  /**
+   * Reorders sprints to the given full list of ids (execution order). Returns
+   * the reordered sprint list (#11).
+   */
+  reorderSprints(projectId: string, orderedIds: string[]): Promise<Sprint[]>;
+  /**
+   * Deletes a sprint. The backend unassigns its tickets (it does NOT delete
+   * them); they fall back to the "no sprint" bucket (#11).
+   */
+  deleteSprint(projectId: string, sprintId: string): Promise<void>;
 }
 
 /**