Move the tariff selection surface (promo-group banner, all-purchased
empty state, and the 1/2-col tariff cards grid with per-tariff CTAs)
into src/components/subscription/purchase/TariffPickerGrid.tsx.
The grid is pure presentation — it owns no queries or mutations.
Two callbacks back into the parent:
- onSelectTariff(tariff) → parent sets selectedTariff + showForm
- onSwitchTariff(tariffId) → parent sets switchTariffId
Internal helpers (formatPrice via useCurrency, applyPromoDiscount
via usePromoDiscount, glass tokens via useTheme + getGlassColors,
navigate for the 'all purchased' back-to-list button) all derive
inside the component — nothing threaded as props beyond data.
SubscriptionPurchase.tsx: 1202 → 908 lines (-294).
