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/bookings/bookings.go

@@ -1588,7 +1588,9 @@ func CreateBookingHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	weekday := int(req.StartTime.Weekday())
+	localStart := req.StartTime.In(londonLocation)
+	// DB uses 0=Monday..6=Sunday; Go uses 0=Sunday..6=Saturday. Convert.
+	weekday := int((localStart.Weekday() + 6) % 7)
 	var closeStr string
 	if err := db.DB.QueryRow(r.Context(), `SELECT end_time::text FROM working_hours WHERE weekday = $1`, weekday).Scan(&closeStr); err != nil {
 		log.Printf("Failed to get hours: %v", err)
@@ -1597,8 +1599,9 @@ func CreateBookingHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	endTime := req.StartTime.Add(time.Duration(svcDuration) * time.Minute)
+	localEnd := localStart.Add(time.Duration(svcDuration) * time.Minute)
 	closeTime, _ := time.Parse("15:04:05", closeStr)
-	if endTime.Hour() > closeTime.Hour() || (endTime.Hour() == closeTime.Hour() && endTime.Minute() > closeTime.Minute()) {
+	if localEnd.Hour() > closeTime.Hour() || (localEnd.Hour() == closeTime.Hour() && localEnd.Minute() > closeTime.Minute()) {
 		http.Error(w, "Cannot book this time - services would extend beyond closing hours", http.StatusBadRequest)
 		return
 	}
@@ -1832,9 +1835,10 @@ func EditBookingHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	weekday := int(req.StartTime.Weekday())
+	// DB uses 0=Monday..6=Sunday; Go uses 0=Sunday..6=Saturday. Convert.
+	weekday := int((req.StartTime.Weekday() + 6) % 7)
 	bookingTime := req.StartTime.Format("15:04:05")
-	daysToMonday := weekday
+	daysToMonday := int(req.StartTime.Weekday())
 	if daysToMonday == 0 {
 		daysToMonday = 7
 	}