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/ports.rs

@@ -1543,6 +1543,12 @@ pub trait IssueStore: Send + Sync {
         expected_version: IssueVersion,
     ) -> Result<(), IssueStoreError>;
 
+    /// Deletes an issue by `#N` reference.
+    ///
+    /// # Errors
+    /// [`IssueStoreError::NotFound`] when absent.
+    async fn delete(&self, root: &ProjectPath, issue_ref: IssueRef) -> Result<(), IssueStoreError>;
+
     /// Reads only the issue carnet projection.
     ///
     /// # Errors