fix(admin): shorten the receipt letter, show the account id

The receipt letter repeated what the receipt itself states. It now carries the
amount, the moment of payment with its offset, and the link — the receipt is
the document, and it opens without authentication, so there is nothing for the
two to disagree about. Subject reworded to name what it is.

The ledger and tax-export tables labelled the account link "card", meaning the
user card. On pages about payments that reads as a bank card, which is exactly
how it was misread. Both now show the first eight characters of the account id,
so the column carries data — two rows from the same buyer are visible at a
glance — while the link still holds the whole id.

backend/internal/inttest/mynalog_test.go

@@ -675,8 +675,22 @@ func TestMyNalogBuyerMails(t *testing.T) {
 	if got := len(box.to(addr)); got != 2 {
 		t.Fatalf("letters after filing = %d, want 2", got)
 	}
-	if !strings.Contains(box.to(addr)[1].Text, "/api/v1/receipt/770000000000/") {
-		t.Errorf("the receipt letter carries no receipt link:\n%s", box.to(addr)[1].Text)
+	receipt := box.to(addr)[1]
+	if receipt.Subject != "Эрудит — Ваш чек покупки" {
+		t.Errorf("receipt letter subject = %q", receipt.Subject)
+	}
+	if !strings.Contains(receipt.Text, "/api/v1/receipt/770000000000/") {
+		t.Errorf("the receipt letter carries no receipt link:\n%s", receipt.Text)
+	}
+	// The amount carries its time-zone offset, because that offset is what decides the tax period
+	// the receipt belongs to and the buyer should see the same moment the receipt states.
+	if !strings.Contains(receipt.Text, "Покупка на сумму: 149.00 ₽ от ") ||
+		!strings.Contains(receipt.Text, "+03:00") {
+		t.Errorf("the receipt letter's amount line is wrong:\n%s", receipt.Text)
+	}
+	// Nothing beyond the amount, the moment and the link: the receipt is the document.
+	if strings.Contains(receipt.Text, "test pack") {
+		t.Errorf("the receipt letter repeats the catalog title; the receipt itself carries it:\n%s", receipt.Text)
 	}
 
 	// 3. The annulment letter after a refund.