Fix: public booking page mixed both Filialen's barbers before any was picked

applyLocationFilter()'s "!selectedLocation" check was meant to mean "only
one Filiale exists, nothing to filter" but also fired in the multi-Filiale
case before the customer had clicked one yet, showing every barber from
every location mixed together in Mitarbeiter. Now keyed on LOCATIONS.length
instead, so with 2+ Filialen nothing shows until one is actually selected.

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

backoffice/app/templates/book.html

@@ -142,7 +142,13 @@
         }
         var visible = [];
         resourceOptionsEl.querySelectorAll('.opt').forEach(function (btn) {
-          var matches = !selectedLocation || btn.getAttribute('data-location-id') === selectedLocation;
+          // With a single Filiale (or none at all) there's nothing to filter
+          // by, same as before Filialen existed. With multiple, a barber only
+          // matches once its own Filiale is actually selected -- before that,
+          // nothing should show (mixing unrelated locations' staff together
+          // would be actively misleading, not just unfiltered).
+          var matches = LOCATIONS.length <= 1
+            || (!!selectedLocation && btn.getAttribute('data-location-id') === selectedLocation);
           btn.hidden = !matches;
           if (matches) {
             visible.push(btn);