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/TicketsPanel.tsx

@@ -18,6 +18,7 @@ import type {
 import { Button, Input, Panel, Spinner, cn } from "@/shared";
 import { useTickets } from "./useTickets";
 import { useProjectAgents } from "./useProjectAgents";
+import { SprintManager } from "./SprintManager";
 import {
   PriorityBadge,
   StatusBadge,
@@ -44,6 +45,7 @@ export function TicketsPanel({ projectId, onOpen }: TicketsPanelProps) {
   const vm = useTickets(projectId);
   const { agents, nameOf } = useProjectAgents(projectId);
   const [showCreate, setShowCreate] = useState(false);
+  const [showSprints, setShowSprints] = useState(false);
   const [newTitle, setNewTitle] = useState("");
   const [newPriority, setNewPriority] = useState<TicketPriority>("medium");
 
@@ -90,6 +92,14 @@ export function TicketsPanel({ projectId, onOpen }: TicketsPanelProps) {
           >
             {showCreate ? "Cancel" : "New"}
           </Button>
+          <Button
+            size="sm"
+            variant="ghost"
+            aria-label="manage sprints"
+            onClick={() => setShowSprints(true)}
+          >
+            Sprints
+          </Button>
           <Button
             size="sm"
             variant="ghost"
@@ -268,6 +278,10 @@ export function TicketsPanel({ projectId, onOpen }: TicketsPanelProps) {
           </Button>
         </div>
       )}
+
+      {showSprints && (
+        <SprintManager vm={vm} onClose={() => setShowSprints(false)} />
+      )}
     </Panel>
   );
 }