Fix "Conflict. Timeslot is taken" raw-JSON error on booking

Customers intermittently hit a full-screen raw JSON error when booking:
{"error":"Conflict. Timeslot is taken or does not fit the service."}

booking_process() re-validates the chosen slot at submit time and returned
409/403 raw JSON. Because the public booking form is a full-page POST, that
JSON filled the whole screen.

The trigger is a double submit. After inserting the booking, booking_process()
synchronously runs two Google Calendar createEvent calls, a lunch sync, an ntfy
push and an SMTP confirmation e-mail before redirecting - several seconds - and
the submit button was never disabled. On mobile the guest taps "Send" again; the
second request arrives after the first has committed, so the slot reads as taken.

Evidence: 168 duplicate booking pairs exist in prod (same guest, slot and worker,
consecutive booking ids, including runs of four). All are from 2025, none from
2026 - the 409 guard added around May 2025 converted those silent duplicates
into today's visible error.

Prevent the double submit:
- disable the submit button and relabel it on first submit, ignore later ones
- add a hidden sendBooking field, since disabling a submit button can drop its
  name/value from the POST and booking_process() bails to the homepage without it

Handle it gracefully when it still happens:
- new _booking_error() renders a localised page in the right skin instead of raw
  JSON, replacing all six JSON responses in booking_process()
- new booking-error views for barber/beauty in no/en/hu, each with a message per
  error case and a link back to booking
- new Service_model::getBookingBySlotAndGuest(); if the guest's own booking for
  that exact slot already exists the submit is a duplicate rather than a real
  conflict, so finish normally instead of erroring. Guarded on a non-empty
  e-mail, as admin block bookings are stored with an empty guest_email.

No schema change. Verified on test, dev and prod: friendly page in all three
languages and both skins, double submit redirects to booking-finished without
creating a duplicate row, and no raw JSON in any response.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TJso3iGT7TkW5tm4RSBohs

application/views/pages/includes/booking-error-form-en.php

@@ -0,0 +1,25 @@
+<?php
+    $bookingErrorMessages = array(
+        'slot_taken' => 'This time slot was taken while you were filling in the form. Please choose another time.',
+        'worker_unavailable' => 'The selected staff member is not working on this day. Please choose another day.',
+        'outside_schedule' => 'The selected time is outside the staff member\'s working hours. Please choose another time.',
+        'alternate_week' => 'The selected staff member only works every second week. Please choose another day.',
+        'after_hours' => 'The treatment would not finish before closing time. Please choose an earlier time.',
+        'too_far' => 'Bookings can only be made up to 3 months in advance.',
+        'default' => 'Something went wrong with your booking. Please try again.',
+    );
+
+    $shownError = isset($bookingErrorCode) && isset($bookingErrorMessages[$bookingErrorCode])
+        ? $bookingErrorMessages[$bookingErrorCode]
+        : $bookingErrorMessages['default'];
+
+    $backSubpage = isset($subpage) && $subpage != '' ? $subpage : 'barber';
+    $backUrl = SITEURL.'en/booking/'.$backSubpage;
+?>
+<div class="bookingContainer" style="min-height:500px;">
+    <div class="bookingThankYouTitle">This time is no longer available</div>
+    <div class="bookingThankYouMessageTitle"><?php echo $shownError;?></div>
+    <div style="text-align:center; margin-top:30px;">
+        <a href="<?php echo $backUrl;?>" style="display:inline-block; padding:12px 28px; background:#000; color:#fff; text-decoration:none; border-radius:5px; font-size:16px;">Back to booking</a>
+    </div>
+</div>