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/Plugin.php

@@ -36,6 +36,13 @@ class Plugin {
 		if ( ! $wpdb instanceof \wpdb ) {
 			return;
 		}
+
+		// Re-run install steps when the plugin files were updated without a fresh
+		// activation (e.g. a deploy), so schema and data migrations still apply.
+		if ( get_option( 'us_schedular_version' ) !== USC_VERSION ) {
+			( new Installer() )->run();
+		}
+
 		$availability     = new AvailabilityRepository( $wpdb );
 		$bookings         = new BookingRepository( $wpdb );
 		$offerings        = new OfferingRepository( $wpdb );