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/AvailabilityController.php

@@ -29,6 +29,18 @@ class AvailabilityController {
 		$slots           = $this->repository->findByInstructor( $instructorId );
 		$offeringChoices = $this->offerings->findAll( $instructorId, Offering::KIND_PRIVATE_LESSON, true );
 
+		// View-state query params only (which view, which week) — nothing is
+		// mutated from them, so no nonce applies.
+		// phpcs:disable WordPress.Security.NonceVerification.Recommended
+		$view          = 'week' === sanitize_key( Val::string( wp_unslash( $_GET['usc_view'] ?? '' ) ) ) ? 'week' : 'list';
+		$requestedWeek = sanitize_text_field( Val::string( wp_unslash( $_GET['usc_week'] ?? '' ) ) );
+		// phpcs:enable WordPress.Security.NonceVerification.Recommended
+
+		$weekStart = WeekCalendar::weekStart( $requestedWeek, Val::int( get_option( 'start_of_week', 1 ) ), current_time( 'Y-m-d' ) );
+		$weekDays  = WeekCalendar::days( $weekStart, $slots );
+		$prevWeek  = ( new \DateTimeImmutable( $weekStart ) )->modify( '-7 days' )->format( 'Y-m-d' );
+		$nextWeek  = ( new \DateTimeImmutable( $weekStart ) )->modify( '+7 days' )->format( 'Y-m-d' );
+
 		include USC_PLUGIN_DIR . 'templates/admin/availability.php';
 	}
 
@@ -58,14 +70,16 @@ class AvailabilityController {
 		$startDt = AvailabilitySlot::normalizeDateTime( sanitize_text_field( Val::string( wp_unslash( $_POST['start_dt'] ?? '' ) ) ) );
 		$endDt   = AvailabilitySlot::normalizeDateTime( sanitize_text_field( Val::string( wp_unslash( $_POST['end_dt'] ?? '' ) ) ) );
 
-		if ( null === $startDt || null === $endDt || $endDt <= $startDt ) {
+		// A window must start and end on the same day (weekly repeat covers longer
+		// ranges) and fit at least one lesson; it is stored as lesson-length slots.
+		if ( null === $startDt || null === $endDt || $endDt <= $startDt || substr( $startDt, 0, 10 ) !== substr( $endDt, 0, 10 ) ) {
 			return;
 		}
 
 		$offeringId = absint( Val::int( $_POST['offering_id'] ?? 0 ) );
 		$duration   = absint( Val::int( $_POST['duration_minutes'] ?? 0 ) );
 
-		$slot = new AvailabilitySlot(
+		$window = new AvailabilitySlot(
 			instructorId:    $instructorId,
 			startDt:         $startDt,
 			endDt:           $endDt,
@@ -73,12 +87,10 @@ class AvailabilityController {
 			offeringId:      $offeringId > 0 ? $offeringId : null,
 		);
 
-		if ( 'weekly' === sanitize_key( Val::string( wp_unslash( $_POST['recurrence'] ?? 'single' ) ) ) ) {
-			$this->repository->createWeeklySeries( $slot, absint( Val::int( $_POST['weeks'] ?? 1 ) ) );
-			return;
-		}
+		$recurrence = sanitize_key( Val::string( wp_unslash( $_POST['recurrence'] ?? 'single' ) ) );
+		$weeks      = absint( Val::int( $_POST['weeks'] ?? 1 ) );
 
-		$this->repository->insert( $slot );
+		$this->repository->createFromWindow( $window, 'weekly' === $recurrence, $weeks );
 		// phpcs:enable WordPress.Security.NonceVerification.Missing
 	}
 }