Move the buy-devices sheet (190 lines of inline JSX + its useQuery +
useMutation) into src/components/subscription/sheets/DeviceTopupSheet.tsx.
Parent passes shared state (purchaseOptions, devicesToAdd, ids); the
sheet self-owns the device-price query and purchase mutation, so the
query only fires when the sheet is actually open.
Subscription.tsx: 2671 → 2477 lines (−194).