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/components/subscription/purchase/TariffPickerGrid.tsx

@@ -137,6 +137,13 @@ export function TariffPickerGrid({
               purchaseOptions &&
               'subscription_is_expired' in purchaseOptions &&
               purchaseOptions.subscription_is_expired === true;
+            // Free (0₽) source tariff: the backend blocks the prorated switch
+            // (free_tariff_cannot_switch) — offer the purchase flow instead.
+            const isOnFreeTariff =
+              isTariffsMode &&
+              purchaseOptions &&
+              'subscription_on_free_tariff' in purchaseOptions &&
+              purchaseOptions.subscription_on_free_tariff === true;
             const canSwitch =
               !isMultiTariff &&
               subscription &&
@@ -144,6 +151,7 @@ export function TariffPickerGrid({
               !isCurrentTariff &&
               !subscription.is_trial &&
               !isSubscriptionExpired &&
+              !isOnFreeTariff &&
               (subscription.is_active || subscription.is_limited);
             const isLegacySubscription =
               subscription && !subscription.is_trial && !subscription.tariff_id;

src/components/subscription/sheets/SwitchTariffSheet.tsx

@@ -27,11 +27,13 @@ import type { Tariff } from '../../../types';
 // ──────────────────────────────────────────────────────────────────
 
 // The backend rejects a switch that must instead go through the purchase flow:
-// the subscription lapsed (`subscription_expired`), or it is a trial that has no
+// the subscription lapsed (`subscription_expired`), it is a trial that has no
 // paid value to prorate and would otherwise be handed a full target period
-// (`trial_cannot_switch`, bug #629889). Both arrive as detail.code +
-// use_purchase_flow=true; some payloads use the legacy `error_code` key, so we
-// accept either.
+// (`trial_cannot_switch`, bug #629889), or it sits on a free 0₽ tariff whose
+// spammed/gifted remainder must reset rather than be prorated and carried
+// (`free_tariff_cannot_switch`, TARIFF_SWITCH_RESET_FREE_DAYS). All arrive as
+// detail.code + use_purchase_flow=true; some payloads use the legacy
+// `error_code` key, so we accept either.
 function shouldUsePurchaseFlow(error: unknown): boolean {
   if (!(error instanceof AxiosError)) return false;
   const detail = error.response?.data?.detail as
@@ -40,7 +42,9 @@ function shouldUsePurchaseFlow(error: unknown): boolean {
   if (!detail || typeof detail !== 'object') return false;
   const code = detail.code ?? detail.error_code;
   return (
-    (code === 'subscription_expired' || code === 'trial_cannot_switch') &&
+    (code === 'subscription_expired' ||
+      code === 'trial_cannot_switch' ||
+      code === 'free_tariff_cannot_switch') &&
     detail.use_purchase_flow === true
   );
 }

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;