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.

.env.example

@@ -30,3 +30,9 @@ AWS_REGION=eu-west-2
 # R2_BUCKET=crussell
 # R2_PUBLIC_URL=https://pub-<your-domain>.r2.dev
 # AWS_REGION=auto
+
+# Square Payment Gateway
+SQUARE_ACCESS_TOKEN=
+SQUARE_LOCATION_ID=
+SQUARE_ENVIRONMENT=mock
+SQUARE_WEBHOOK_SIGNATURE_KEY=