feat: implement hx-boost for reliable loading indication

src/Service/BookingService.php

@@ -93,9 +93,6 @@ class BookingService
      */
     public function saveBookingDto(Request $request, BookingDto $bookingDto, string $mode): void
     {
-        // Track last session update for staleness detection
-        $bookingDto->lastSessionUpdate = new \DateTimeImmutable();
-
         $sessionKey = $this->getSessionKey($mode);
         $request->getSession()->set($sessionKey, $bookingDto);
     }