feat: Square payment integration, booking flow redesign, and timezone/weekday fixes

- Add Square payment integration (mock + handlers + UI): terminal/online payments,
  refunds, tips, saved cards, webhooks. Build-tagged dev/prod clients.
- Redesign booking flow: Step 4 conditional (deposit only), Step 5 confirmation
  screen with booking ID, auto-submit on transition.
- Redesign schedule modal: 2x3 button grid with Pay Deposit/Pay Early logic.
- Add deposit warning banner at Step 1 for users with outstanding deposits.
- Fix weekday conversion bug: Go 0=Sunday vs DB 0=Monday mismatch in 6 locations.
- Fix timezone bug: UTC vs London time in closing hours validation.
- Fix frontend error parsing: plain text backend errors now displayed correctly.
- Fix crypto.randomUUID fallback for environments without Web Crypto.
- Add 7 new regression tests: closing hours, advance check, active booking limit,
  weekday conversion, UTC/London, deposit snapshot, exceptional hours.
- Fix 3 flaky tests: dynamic dates instead of fixed, no-show timing.

backend/handlers/payments/validators.go

@@ -0,0 +1,44 @@
+package payments
+
+import "errors"
+
+// Valid payment types
+var validPaymentTypes = map[string]bool{
+	"deposit": true,
+	"full":    true,
+	"tip":     true,
+	"balance": true,
+	"partial": true,
+}
+
+// ValidateAmount checks that amount is greater than 0
+func ValidateAmount(amount int64) error {
+	if amount <= 0 {
+		return errors.New("amount must be greater than 0")
+	}
+	return nil
+}
+
+// ValidatePaymentType checks that payment type is valid
+func ValidatePaymentType(pt string) error {
+	if !validPaymentTypes[pt] {
+		return errors.New("invalid payment type")
+	}
+	return nil
+}
+
+// ValidateRefundReason checks that refund reason is not empty
+func ValidateRefundReason(reason string) error {
+	if reason == "" {
+		return errors.New("refund reason is required")
+	}
+	return nil
+}
+
+// ValidateCardInfo checks that at least one of cardID or newCardToken is provided
+func ValidateCardInfo(cardID, newCardToken *string) error {
+	if cardID == nil && (newCardToken == nil || *newCardToken == "") {
+		return errors.New("either card_id or new_card_token is required")
+	}
+	return nil
+}