test(frontend): cover NewSessionDialog model degradation + invalidate sessions on resume

- Add NewSessionDialog.test.tsx covering §9 graceful degradation:
  empty models -> free-text input; non-empty models -> dropdown; plus a
  createSession body-keys assertion
- Normalize model entries to {value,label} so object-shaped models
  ({provider,id}) render correctly in the dropdown (was assuming strings)
- NavSessions: invalidate the ["sessions"] query after a successful
  resumeSession so the list/status refreshes immediately

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

frontend/src/shell/NavSessions.tsx

@@ -1,5 +1,5 @@
 // frontend/src/shell/NavSessions.tsx
-import { useQuery } from "@tanstack/react-query";
+import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { listSessions } from "../api/sessions";
 import { resumeSession } from "../api/sessions";
 import type { SessionSummary } from "../api/types";
@@ -10,6 +10,7 @@ interface Props {
 }
 
 export function NavSessions({ activeSessionId, onSessionSelect }: Props) {
+  const queryClient = useQueryClient();
   const { data: sessions = [] } = useQuery<SessionSummary[]>({
     queryKey: ["sessions"],
     queryFn: listSessions,
@@ -18,6 +19,7 @@ export function NavSessions({ activeSessionId, onSessionSelect }: Props) {
 
   async function handleClick(id: string) {
     await resumeSession(id);
+    queryClient.invalidateQueries({ queryKey: ["sessions"] });
     onSessionSelect(id);
   }