Add massage as a third vertical, driven by a config registry

Introduces /massage alongside barber and beauty: landing page, booking
flow, admin support, home tile and SEO entries, in all three languages.

Architecture
- application/config/verticals.php + vertical_helper.php: one registry
  entry per vertical (branding, assets, views, behaviour flags). A fourth
  vertical is a config entry plus content files.
- Strangler: barber and beauty keep pointing at their existing view files,
  so their rendered HTML is unchanged. Only massage uses the new generic
  pages/vertical-*.php and includes/vertical-*.php views, which collapse
  the four duplicated per-language nav/footer branches into one.
- Pages::vertical() + one route; booking(), booking_finished(),
  _booking_error() and manage_booking() are now registry-driven.

Worker/vertical coupling
- getActiveWorkers() derives the vertical from services.service_category_id
  instead of the workers.is_barber / is_beauty flags, which were a
  hand-maintained cache of exactly that fact. Verified against production
  data: the derived set reproduced the stored flags for every worker, in
  both verticals. No schema change was needed for massage.
- The legacy flags are now written through from the category so a rollback
  cannot strand a new worker, and the admin worker UI shows the derived
  verticals read-only instead of two dropdowns that controlled nothing.

Bug fixes found along the way (all pre-existing)
- booking_process() had no server-side category guard; cross-vertical
  mixing was prevented only by client-side JS.
- add-service-form / add-worker-form emitted `selected` on every category
  option, so the newest category silently became the default.
- update-service-form offered only barber/beauty, so editing a service of
  any other type silently rewrote it.
- getWorkerScheduleByDay ignored schedule overrides while getAvailableTimes
  honoured them, so slots could be shown and then rejected. Added an
  override-aware getWorkerScheduleForDate() and used it in both guards.
- Booking lists dereferenced a null service if one had been hard-deleted.
- main.css: .tiles was tuned for exactly two tiles, including an
  absolutely-positioned .style1 at the 1280px breakpoint.

Massage-specific behaviour, opt-in per vertical
- strip_category_prefix: grouped service lists show "50 min" under the
  treatment heading rather than repeating the full name. The full name is
  carried in data-service-name so the totals panel stays unambiguous, and
  services.service_name is untouched for emails and admin.
- single_service_booking: one treatment per booking, enforced in the UI and
  in booking_process(). Re-clicking the selection releases it.
- Displayed treatment time (50/80/110 min) is in the service name; the
  booked slot (60/90/120 min) is service_time and covers changing and
  payment. service_time is never shown to the guest.

DB migrations for dev/prod are in documents/ - additive only, no ALTER.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

application/views/pages/includes/booking-error-form-no.php

@@ -6,6 +6,9 @@
         'alternate_week' => 'Den valgte medarbeideren jobber kun annenhver uke. Vennligst velg en annen dag.',
         'after_hours' => 'Behandlingen rekker ikke å bli ferdig innen stengetid. Vennligst velg et tidligere tidspunkt.',
         'too_far' => 'Du kan kun bestille time inntil 3 måneder frem i tid.',
+        'category_mismatch' => 'Den valgte medarbeideren utfører ikke de valgte tjenestene. Vennligst velg på nytt.',
+        'single_service_only' => 'Du kan kun bestille én behandling om gangen. Vennligst velg kun én.',
+        'no_service' => 'Du har ikke valgt noen tjeneste. Vennligst velg minst én tjeneste.',
         'default' => 'Noe gikk galt med bestillingen. Vennligst prøv igjen.',
     );