Fix countdown timer using server's next_check_at

- Add next_check_at to Product interface
- Use server-calculated next check time instead of client-side estimate
- Accounts for server-side jitter (±5 min) correctly

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -59,6 +59,7 @@ export interface Product {
   image_url: string | null;
   refresh_interval: number;
   last_checked: string | null;
+  next_check_at: string | null;
   stock_status: StockStatus;
   price_drop_threshold: number | null;
   target_price: number | null;