The bot backend now rejects prorated tariff switches from a free (0RUB)
tariff with 400 free_tariff_cannot_switch + use_purchase_flow=true
(TARIFF_SWITCH_RESET_FREE_DAYS: the free remainder must reset, not be
prorated over hundreds of accumulated days and carried to a paid
tariff). Handle it like the existing trial_cannot_switch contract:
- SwitchTariffSheet: accept free_tariff_cannot_switch in
shouldUsePurchaseFlow so the sheet hands off to the purchase form
- TariffPickerGrid: new subscription_on_free_tariff purchase-options
flag disables the switch CTA up front and shows purchase instead
Companion to BEDOLAGA-DEV/remnawave-bedolaga-telegram-bot#3056.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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