feat(tickets): suppression d'un ticket (backend + frontend)

Ajoute la suppression complète d'un ticket (#6).

Backend Rust :
- port IssueStore::delete (NotFound si absent) + event IssueDeleted{freed_sprint}
- FsIssueStore::delete : supprime .ideai/tickets/<N>/ et l'index sous lock
- use case DeleteIssue + adaptations des tests sprint/ticket_assistant au port
- commande Tauri ticket_delete + câblage events/state/lib

Frontend :
- gateway delete (ports, adapter ticket + mock, domain)
- useTicketDetail : retrait de la liste et fermeture du détail via event issueDeleted
- intégration TicketDetail + tests

Tests verts : application/issue_usecases (6), infrastructure/issue_store (7),
app-tauri --lib (56), frontend vitest (503), npm run build (exit 0).

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

crates/domain/src/events.rs

@@ -317,6 +317,15 @@ pub enum DomainEvent {
         /// New optimistic version.
         version: IssueVersion,
     },
+    /// An issue was deleted.
+    IssueDeleted {
+        /// Owning project.
+        project_id: ProjectId,
+        /// Human-friendly issue reference.
+        issue_ref: IssueRef,
+        /// Sprint membership released by the deletion, when any.
+        freed_sprint: Option<SprintId>,
+    },
     /// An issue status changed.
     IssueStatusChanged {
         /// Human-friendly issue reference.