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/testutils/testdb/testdb.go

@@ -99,7 +99,11 @@ func Migrate(t *testing.T, pool *pgxpool.Pool) {
 			"user_notification_preferences",
 			"user_referrals",
 			"booking_services",
+			"refunds",
 			"payments",
+			"user_saved_cards",
+			"square_deposits",
+			"affiliate_payouts",
 			"bookings",
 			"user_patch_tests",
 			"patch_tests",
@@ -202,7 +206,11 @@ func TruncateTables(t *testing.T, pool *pgxpool.Pool) {
         "user_social_logins",
         "verification_codes",
         "booking_services",
+        "refunds",
         "payments",
+        "user_saved_cards",
+        "square_deposits",
+        "affiliate_payouts",
         "bookings",
         "booking_edit_requests",
         "user_patch_tests",