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/AvailabilityRepository.php

@@ -30,6 +30,26 @@ class AvailabilityRepository {
 		return $this->db->insert_id;
 	}
 
+	/**
+	 * Persist an availability window as individually bookable lesson-length slots.
+	 * The window is split into consecutive `duration_minutes` chunks; each chunk
+	 * becomes its own row (and, when weekly, its own weekly series) so students can
+	 * book any open lesson-length block within the window.
+	 *
+	 * @return list<int> Inserted slot IDs.
+	 */
+	public function createFromWindow( AvailabilitySlot $window, bool $weekly = false, int $weeks = 1 ): array {
+		$ids = [];
+
+		foreach ( $window->splitByDuration() as $slot ) {
+			$ids = $weekly
+				? array_merge( $ids, $this->createWeeklySeries( $slot, $weeks ) )
+				: [ ...$ids, $this->insert( $slot ) ];
+		}
+
+		return $ids;
+	}
+
 	/**
 	 * Create a weekly-recurring series from a template slot. Each occurrence is a
 	 * separate row one week apart, all sharing a `recurrence_group` (the id of the
@@ -87,8 +107,10 @@ class AvailabilityRepository {
 	 * @return list<AvailabilitySlot>
 	 */
 	public function findAvailable( int $instructorId = 0, int $offeringId = 0, int $durationMinutes = 0, string $from = '', string $to = '' ): array {
-		$where  = [ 'is_booked = 0' ];
-		$params = [];
+		// A slot whose start has passed can no longer be booked, so it is never
+		// "available" regardless of the requested range.
+		$where  = [ 'is_booked = 0', 'start_dt >= %s' ];
+		$params = [ current_time( 'mysql' ) ];
 
 		if ( $instructorId > 0 ) {
 			$where[]  = 'instructor_id = %d';
@@ -189,6 +211,47 @@ class AvailabilityRepository {
 		return 1 === $updated;
 	}
 
+	/**
+	 * One-time upgrade for rows created before windows were split on save: a
+	 * window stored as a single row (e.g. 09:00–16:00 with 60-minute lessons)
+	 * showed to students as one giant slot. Rewrites every unbooked same-day
+	 * window longer than its lesson length as lesson-length rows: the original
+	 * row is trimmed to the first chunk (keeping its id and any recurrence
+	 * group), and the remaining chunks are inserted as one-off rows.
+	 */
+	public function splitOversizedWindows(): void {
+		$rows = $this->db->get_results(
+			$this->db->prepare(
+				'SELECT * FROM %i
+                 WHERE is_booked = 0
+                   AND DATE(start_dt) = DATE(end_dt)
+                   AND TIMESTAMPDIFF(MINUTE, start_dt, end_dt) > duration_minutes',
+				$this->table
+			)
+		);
+
+		foreach ( $rows ?? [] as $row ) {
+			$window = AvailabilitySlot::fromRow( $row );
+			$chunks = $window->splitByDuration();
+
+			if ( [] === $chunks ) {
+				continue;
+			}
+
+			$this->db->update(
+				$this->table,
+				[ 'end_dt' => $chunks[0]->endDt ],
+				[ 'id' => $window->id ],
+				[ '%s' ],
+				[ '%d' ]
+			);
+
+			foreach ( array_slice( $chunks, 1 ) as $chunk ) {
+				$this->insert( $chunk );
+			}
+		}
+	}
+
 	/**
 	 * Delete an unbooked slot. Returns false if the slot is already booked.
 	 */