feat(payments): send no fiscal receipt; keep the code switchable

The merchant accepts payments as a sole proprietor on НПД, which is outside
54-ФЗ: there is no online cash register, YooKassa does not serve receipts for
that regime at all (checked with their support), and each operation is reported
by the merchant to «Мой налог», which issues the чек. So no `receipt` is sent
with a payment or a refund.

This also removes a failure class rather than just code: a malformed receipt
was an API error at payment creation, which broke the purchase outright.

The fiscal code is kept dormant rather than deleted. All of it now sits behind
one switch, `BACKEND_YOOKASSA_VAT_CODE`: unset — the default — builds and sends
nothing; a 54-ФЗ rate code turns «Чеки от ЮKassa» back on unchanged. The return
is foreseeable, which is why the switch exists: НПД carries an annual income
ceiling, and losing the regime puts 54-ФЗ back in force, at which point this is
a deploy-variable edit instead of writing the integration again.

The D36 email anchor still gates a direct purchase. It had two justifications —
a recovery anchor and the receipt address — and only the second is gone; without
an email a paying customer who loses the account loses the chips with it. What
was missing is that the rule was enforced but never communicated: the wallet
showed the packs to a player signed in through VK or Telegram in a browser, and
tapping Buy produced a bare "something went wrong". It now says "add an email in
your profile" in the buy tab instead, linking to the profile; spending
already-earned chips is untouched. The predicate is a pure function so it is
covered by the node-env unit tests rather than needing a browser.

Tests: the receipt-off default is pinned by an integration test asserting a
purchase carries no receipt, and the dormant path by one that switches a VAT
code on and checks the receipt reappears with the right fiscal attributes;
plus unit coverage for the enable predicate and the wallet's email rule.

A note for whoever runs the numbers next: the shared (svelte + i18n) chunk is
now 40 bytes under its 31 KB gzip budget.

Decision D51 revised.

backend/internal/yookassa/receipt.go

@@ -1,9 +1,14 @@
 package yookassa
 
-// Fiscal receipt attributes for «Чеки от ЮKassa» (54-ФЗ). YooKassa registers the receipt itself —
-// no cash register to rent, no OFD contract — but only if the payment and refund requests carry a
-// receipt object, so unlike the previous rail's cabinet-side receipts these fields are load-bearing
-// code. Reference:
+// Fiscal receipt support for «Чеки от ЮKassa» (54-ФЗ). YooKassa registers a receipt itself — no cash
+// register to rent, no OFD contract — but only if the payment and refund requests carry a receipt
+// object.
+//
+// It is DORMANT: the merchant runs on НПД, which is outside 54-ФЗ, so no receipt is sent and the
+// merchant reports each operation to «Мой налог» instead. The code stays because the switch back is
+// foreseeable — НПД has an annual income ceiling, and losing the regime puts 54-ФЗ back in force —
+// and turning it on is then setting one deploy variable rather than writing this again. Everything
+// here is inert while BACKEND_YOOKASSA_VAT_CODE is unset (see ReceiptEnabled). Reference:
 // https://yookassa.ru/developers/payment-acceptance/receipts/54fz/yoomoney/parameters-values
 const (
 	// PaymentSubjectService is the settlement subject (54-ФЗ tag 1212) — what the money is taken
@@ -15,10 +20,13 @@ const (
 	PaymentModeFullPayment = "full_payment"
 
 	// VatCodeNone is the VAT rate code (54-ФЗ tag 1199) for «Без НДС» — the rate a sole proprietor on
-	// УСН or ПСН charges. It is the default; the deployed value is configurable because the rate is
-	// the one receipt attribute that genuinely changes (Russia raised the main rate on 1 Jan 2026,
-	// adding codes 11 and 12).
+	// УСН or ПСН charges. The deployed value is configurable because the rate is the one receipt
+	// attribute that genuinely changes (Russia raised the main rate on 1 Jan 2026, adding codes 11
+	// and 12).
 	VatCodeNone = 1
+	// VatCodeOff is the configured value that means "send no receipt at all" — the state a merchant
+	// outside 54-ФЗ runs in. It is the default, so receipts stay off until a rate is deliberately set.
+	VatCodeOff = 0
 	// VatCodeMax is the highest VAT rate code the API accepts; codes run 1..12.
 	VatCodeMax = 12
 )
@@ -78,5 +86,10 @@ func TruncateDescription(s string) string {
 	return string(r[:maxDescriptionRunes])
 }
 
-// ValidVatCode reports whether code is within the range the API accepts (1..12).
-func ValidVatCode(code int) bool { return code >= 1 && code <= VatCodeMax }
+// ValidVatCode reports whether code is a rate the API accepts (1..12) or VatCodeOff, which disables
+// receipts altogether.
+func ValidVatCode(code int) bool { return code == VatCodeOff || (code >= 1 && code <= VatCodeMax) }
+
+// ReceiptEnabled reports whether a configured VAT rate code asks for fiscal receipts. A merchant
+// outside 54-ФЗ leaves the code unset and no receipt is built or sent.
+func ReceiptEnabled(vatCode int) bool { return vatCode != VatCodeOff }