feat(tickets): attribution d'un sprint à la création via popup SprintPicker (#38)

Nouveau composant SprintPicker permettant de choisir un sprint lors de
la création d'un ticket depuis TicketsPanel ; export ajouté à l'index
des features tickets. Frontend-pur.

QA vert : tsc --noEmit exit 0, vitest 63 fichiers / 634 tests passés.

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

frontend/src/features/tickets/index.ts

@@ -15,6 +15,8 @@ export type {
   TicketPickerResult,
   TicketPickerSelectionMode,
 } from "./TicketPicker";
+export { SprintPicker } from "./SprintPicker";
+export type { SprintPickerProps } from "./SprintPicker";
 export { TicketFacetsBar } from "./TicketFacetsBar";
 export type { TicketFacetsBarProps } from "./TicketFacetsBar";
 export { useTicketSearch } from "./useTicketSearch";