feat: htmx powered requests, improved loading indicator

feat: loading indicator and htmx form submit

src/Form/Model/BookingDto.php

@@ -58,6 +58,12 @@ class BookingDto
      */
     public ?Booking $booking = null;
 
+    /**
+     * Timestamp of last session update (for staleness detection).
+     * Updated automatically by BookingService::saveBookingDto().
+     */
+    public ?\DateTimeImmutable $lastSessionUpdate = null;
+
     public function __construct(public Travel $travel, public int $hotelId)
     {
     }