Add per-embed lesson-type and section options to the booking block

Three sidebar options on the Lesson Booking block, all mirrored as shortcode
attributes and carried to the front end as data attributes on
#us-booking-app (or as omitted containers):

- Lesson type (lessonTypeId / lesson_type) pins the calendar to a single
  private-lesson type: only the times bookable as it are listed, and it is
  the only type bookable there, auto-selected on the registration form. A
  pinned type that is no longer offered says so instead of showing an empty
  calendar.
- Show the lesson-type filter (showTypeFilter / show_filter) drops the
  "Show Only" control for studios that do not want it.
- Sections (displayMode / show) embeds one half of the page — the booking
  calendar or the student's upcoming lessons — so the two can live on
  different pages. The script skips the work belonging to a missing half:
  no availability or catalog request for an upcoming-only embed, no
  bookings request for a booking-only one. An unrecognised value renders
  the whole page. The editor preview follows the same setting.

Also fixes the expanded filter's first lesson type sharing a line with the
"Lesson type" heading — the choices now sit in their own row beneath it.

Co-Authored-By: Claude Opus 5 <[email protected]>

assets/css/frontend.css

@@ -141,10 +141,6 @@
 }
 
 .us-type-filter {
-    display: flex;
-    flex-wrap: wrap;
-    align-items: center;
-    gap: 8px 16px;
     margin-bottom: 12px;
     padding: 8px 12px;
     border: 1px solid #eee;
@@ -152,9 +148,18 @@
 }
 
 .us-type-filter-heading {
+    display: block;
+    margin-bottom: 6px;
     font-weight: 600;
 }
 
+.us-type-filter-choices {
+    display: flex;
+    flex-wrap: wrap;
+    align-items: center;
+    gap: 8px 16px;
+}
+
 .us-type-filter-choice {
     display: inline-flex;
     align-items: center;