fix(subscription): fall back to purchase flow for free-tariff switches

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>

src/types/index.ts

@@ -364,6 +364,9 @@ export interface TariffsPurchaseOptions {
   // New fields for expired subscription handling
   subscription_status?: string;
   subscription_is_expired?: boolean;
+  // Free (0₽) source tariff: switch is blocked (free days must reset),
+  // tariff cards must offer the purchase flow instead of the prorated switch
+  subscription_on_free_tariff?: boolean;
   has_subscription?: boolean;
   // Multi-tariff: all available tariffs already purchased
   all_tariffs_purchased?: boolean;