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/WeekCalendar.php

@@ -0,0 +1,57 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Availability;
+
+/**
+ * Pure helpers for the weekly calendar views: resolving which week to show and
+ * bucketing slots into that week's seven days.
+ */
+class WeekCalendar {
+
+	/**
+	 * Resolve a requested week anchor to the date of the first day of its week.
+	 * `$requested` may be any date (`Y-m-d`) inside the wanted week; anything
+	 * unparseable falls back to `$today`. `$startOfWeek` follows WordPress's
+	 * `start_of_week` option (0 = Sunday … 6 = Saturday).
+	 */
+	public static function weekStart( string $requested, int $startOfWeek, string $today ): string {
+		$anchor = self::parseDay( $requested ) ?? self::parseDay( $today ) ?? new \DateTimeImmutable( 'today' );
+		$shift  = ( (int) $anchor->format( 'w' ) - $startOfWeek + 7 ) % 7;
+
+		return $anchor->modify( '-' . $shift . ' days' )->format( 'Y-m-d' );
+	}
+
+	/**
+	 * Bucket slots into the seven days of the week starting at `$weekStart`
+	 * (`Y-m-d`). Every day is present, empty or not, in calendar order.
+	 *
+	 * @param list<AvailabilitySlot> $slots
+	 * @return list<array{date: string, slots: list<AvailabilitySlot>}>
+	 */
+	public static function days( string $weekStart, array $slots ): array {
+		$start = self::parseDay( $weekStart ) ?? new \DateTimeImmutable( 'today' );
+
+		$byDay = [];
+		foreach ( $slots as $slot ) {
+			$byDay[ substr( $slot->startDt, 0, 10 ) ][] = $slot;
+		}
+
+		$days = [];
+		for ( $i = 0; $i < 7; $i++ ) {
+			$date   = $start->modify( '+' . $i . ' days' )->format( 'Y-m-d' );
+			$days[] = [
+				'date'  => $date,
+				'slots' => $byDay[ $date ] ?? [],
+			];
+		}
+
+		return $days;
+	}
+
+	private static function parseDay( string $value ): ?\DateTimeImmutable {
+		$day = \DateTimeImmutable::createFromFormat( '!Y-m-d', $value );
+
+		return false !== $day && $day->format( 'Y-m-d' ) === $value ? $day : null;
+	}
+}