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/internal/square/square.go

@@ -0,0 +1,49 @@
+//go:build !dev
+// +build !dev
+
+package square
+
+import (
+	"context"
+	"errors"
+)
+
+var Client SquareClient
+
+type ProdClient struct{}
+
+func NewClient() SquareClient {
+	return NewProdClient()
+}
+
+func NewProdClient() SquareClient {
+	return &ProdClient{}
+}
+
+func (p *ProdClient) CreatePayment(ctx context.Context, req CreatePaymentReq) (*PaymentResult, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) CreateCheckout(ctx context.Context, req CreateCheckoutReq) (*CheckoutResult, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) GetCheckout(ctx context.Context, checkoutID string) (*PaymentResult, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) RefundPayment(ctx context.Context, req RefundPaymentReq) (*RefundResult, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) CreateCardOnFile(ctx context.Context, userID, cardToken string) (*CardOnFile, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) GetCardsOnFile(ctx context.Context, userID string) ([]CardOnFile, error) {
+	return nil, errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}
+
+func (p *ProdClient) DeleteCardOnFile(ctx context.Context, cardID string) error {
+	return errors.New("square payments not yet configured — set SQUARE_ACCESS_TOKEN and SQUARE_LOCATION_ID in .env")
+}