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/Installer.php

@@ -4,11 +4,13 @@ declare(strict_types=1);
 namespace Unsupervised\Schedular;
 
 use Unsupervised\Schedular\Auth\RoleManager;
+use Unsupervised\Schedular\Availability\AvailabilityRepository;
 
 class Installer {
 
 	public function run(): void {
 		$this->createTables();
+		$this->migrateData();
 		( new RoleManager() )->createRoles();
 		flush_rewrite_rules();
 		update_option( 'us_schedular_version', USC_VERSION );
@@ -27,4 +29,13 @@ class Installer {
 			dbDelta( $sql );
 		}
 	}
+
+	private function migrateData(): void {
+		global $wpdb;
+		if ( ! $wpdb instanceof \wpdb ) {
+			return;
+		}
+
+		( new AvailabilityRepository( $wpdb ) )->splitOversizedWindows();
+	}
 }