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/application/tests/sprint_usecases.rs

@@ -159,6 +159,19 @@ impl IssueStore for FakeIssues {
         Ok(())
     }
 
+    async fn delete(
+        &self,
+        _root: &ProjectPath,
+        issue_ref: IssueRef,
+    ) -> Result<(), IssueStoreError> {
+        self.issues
+            .lock()
+            .unwrap()
+            .remove(&issue_ref)
+            .map(|_| ())
+            .ok_or(IssueStoreError::NotFound)
+    }
+
     async fn read_carnet(
         &self,
         _root: &ProjectPath,