refactor(vibe): simplify durable session flow

frontend/src/types.ts

@@ -75,8 +75,8 @@ export interface HealthResponse {
   redis: 'ok' | 'error' | 'unknown';
 }
 
-// Active v2 recommendation session. The sessionId comes from
-// POST /api/v2/vibe/start and identifies the Redis-stored plan.
+// Active durable recommendation session. The sessionId comes from
+// POST /api/v2/vibe/sessions and identifies its persisted plan.
 export interface VibeSession {
   sessionId: string;
   seedTrackId: string | null;