feat: implement hx-boost for reliable loading indication

src/Form/Model/BookingDto.php

@@ -60,12 +60,6 @@ class BookingDto
      */
     public ?Booking $booking = null;
 
-    /**
-     * Timestamp of last session update (for staleness detection).
-     * Updated automatically by BookingService::saveBookingDto().
-     */
-    public ?\DateTimeImmutable $lastSessionUpdate = null;
-
     /**
      * Fingerprint of the booking state when loaded from API (edit mode only).
      * This property stores the original state and is never updated after initial load.