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/AvailabilitySlot.php

@@ -37,6 +37,42 @@ class AvailabilitySlot {
 		return null;
 	}
 
+	/**
+	 * Split this window into consecutive lesson-length slots: 09:00–16:00 with
+	 * 60-minute lessons yields seven bookable slots. A trailing remainder shorter
+	 * than the lesson length is dropped, and an empty list is returned when the
+	 * window cannot fit a single lesson.
+	 *
+	 * @return list<self>
+	 */
+	public function splitByDuration(): array {
+		if ( $this->durationMinutes <= 0 ) {
+			return [];
+		}
+
+		$end  = new \DateTimeImmutable( $this->endDt );
+		$step = new \DateInterval( 'PT' . $this->durationMinutes . 'M' );
+
+		$cursor   = new \DateTimeImmutable( $this->startDt );
+		$chunkEnd = $cursor->add( $step );
+
+		$slots = [];
+		while ( $chunkEnd <= $end ) {
+			$slots[] = new self(
+				instructorId:    $this->instructorId,
+				startDt:         $cursor->format( 'Y-m-d H:i:s' ),
+				endDt:           $chunkEnd->format( 'Y-m-d H:i:s' ),
+				durationMinutes: $this->durationMinutes,
+				offeringId:      $this->offeringId,
+			);
+
+			$cursor   = $chunkEnd;
+			$chunkEnd = $cursor->add( $step );
+		}
+
+		return $slots;
+	}
+
 	public static function fromRow( \stdClass $row ): self {
 		return new self(
 			instructorId:    Val::int( $row->instructor_id ),