Move the tariff-switch preview modal (137 lines of inline JSX
+ its previewTariffSwitch query + switchTariff mutation + the
auto-scroll-into-view useEffect + the ref) into a self-contained
sheet at src/components/subscription/sheets/SwitchTariffSheet.tsx.
Parent keeps the trigger (switchTariffId selection state lives on
the TariffPickerGrid card click) but no longer holds the query, the
mutation, the ref, or the side-effect. The subscription_expired
backend signal — which used to call three setters at parent level
(setSelectedTariff, setSelectedTariffPeriod, setShowTariffPurchase)
— now flows through a single onExpiredFallback(tariff) callback.
SubscriptionPurchase.tsx: 2010 → 1841 lines (-169).
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