feat: caret preservation and Luhn validation in card inputs

- Added generic formatAndPreserveCursor() helper on frontend to track
  and restore selection caret position during dynamic input sanitization
- Applied to all card inputs, gift card code inputs, and expiry inputs
- Added Luhn validation (isValidLuhn) for saved cards and gift cards
- Rebuilt payments test DB and got 100% green tests

backend/testutils/testdb/testdb.go

@@ -105,6 +105,8 @@ func Migrate(t *testing.T, pool *pgxpool.Pool) {
 			"square_deposits",
 			"affiliate_payouts",
 			"financial_aggregates",
+			"gift_cards",
+			"user_giftcard_balances",
 			"bookings",
 			"user_patch_tests",
 			"patch_tests",
@@ -213,6 +215,8 @@ func TruncateTables(t *testing.T, pool *pgxpool.Pool) {
         "square_deposits",
         "affiliate_payouts",
         "financial_aggregates",
+        "gift_cards",
+        "user_giftcard_balances",
         "bookings",
         "booking_edit_requests",
         "user_patch_tests",