Split availability windows into bookable lesson-length slots with weekly calendar views

Availability windows were stored and served as a single bookable row, so a
9:00 AM-4:00 PM window showed to students as one giant slot and booking it
consumed the whole day; past and multi-day windows also leaked into the
booking page as nonsense entries.

- Split windows into consecutive lesson-length slots on save (REST and admin
  form); each chunk is independently bookable and weekly recurrence creates a
  series per chunk so "reserve this time weekly" holds the same hour each week
- Reject windows spanning multiple days or shorter than the lesson length
  (400 invalid_window)
- Never return slots whose start has passed from GET /availability
- Migrate pre-split rows: Plugin::boot re-runs the Installer on version change
  and AvailabilityRepository::splitOversizedWindows() rewrites unbooked
  same-day oversized windows in place
- Display all times in 12-hour AM/PM form (booking page, wp-admin lists,
  editor previews)
- Add a List | Week view toggle to the student booking page and the
  instructor availability page, with previous/next-week navigation honouring
  the site's start_of_week option (new WeekCalendar helper)

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

src/Availability/AvailabilityEndpoint.php

@@ -133,7 +133,11 @@ class AvailabilityEndpoint {
 			return new \WP_Error( 'invalid_datetime', __( 'Provide a valid start and end, with the end after the start.', 'unsupervised-schedular' ), [ 'status' => 400 ] );
 		}
 
-		$slot = new AvailabilitySlot(
+		if ( substr( $startDt, 0, 10 ) !== substr( $endDt, 0, 10 ) ) {
+			return new \WP_Error( 'invalid_window', __( 'Availability must start and end on the same day. Use the weekly repeat to cover multiple weeks.', 'unsupervised-schedular' ), [ 'status' => 400 ] );
+		}
+
+		$window = new AvailabilitySlot(
 			instructorId:    $instructorId,
 			startDt:         $startDt,
 			endDt:           $endDt,
@@ -141,15 +145,17 @@ class AvailabilityEndpoint {
 			offeringId:      $offeringId > 0 ? $offeringId : null,
 		);
 
-		if ( 'weekly' === $request->get_param( 'recurrence' ) ) {
-			$ids = $this->repository->createWeeklySeries( $slot, absint( Val::int( $request->get_param( 'weeks' ) ) ) );
-
-			return new \WP_REST_Response( [ 'ids' => $ids ], 201 );
+		if ( [] === $window->splitByDuration() ) {
+			return new \WP_Error( 'invalid_window', __( 'The availability window is shorter than the lesson length.', 'unsupervised-schedular' ), [ 'status' => 400 ] );
 		}
 
-		$id = $this->repository->insert( $slot );
+		$ids = $this->repository->createFromWindow(
+			$window,
+			'weekly' === $request->get_param( 'recurrence' ),
+			absint( Val::int( $request->get_param( 'weeks' ) ) )
+		);
 
-		return new \WP_REST_Response( [ 'id' => $id ], 201 );
+		return new \WP_REST_Response( [ 'ids' => $ids ], 201 );
 	}
 
 	public function delete( \WP_REST_Request $request ): \WP_REST_Response|\WP_Error {