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]>

tests/Unit/Availability/WeekCalendarTest.php

@@ -0,0 +1,66 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Availability;
+
+use Unsupervised\Schedular\Availability\AvailabilitySlot;
+use Unsupervised\Schedular\Availability\WeekCalendar;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class WeekCalendarTest extends TestCase
+{
+    public function testWeekStartShiftsBackToConfiguredStartOfWeek(): void
+    {
+        // 2026-07-08 is a Wednesday; with Monday (1) starts, the week begins Jul 6.
+        self::assertSame('2026-07-06', WeekCalendar::weekStart('2026-07-08', 1, '2026-01-01'));
+
+        // With Sunday (0) starts, the same Wednesday's week begins Jul 5.
+        self::assertSame('2026-07-05', WeekCalendar::weekStart('2026-07-08', 0, '2026-01-01'));
+    }
+
+    public function testWeekStartIsIdempotentWhenAnchorIsAlreadyTheStart(): void
+    {
+        self::assertSame('2026-07-06', WeekCalendar::weekStart('2026-07-06', 1, '2026-01-01'));
+    }
+
+    public function testWeekStartFallsBackToTodayForInvalidInput(): void
+    {
+        // 2026-07-05 is a Sunday; with Monday starts, its week began Jun 29.
+        self::assertSame('2026-06-29', WeekCalendar::weekStart('', 1, '2026-07-05'));
+        self::assertSame('2026-06-29', WeekCalendar::weekStart('not-a-date', 1, '2026-07-05'));
+        self::assertSame('2026-06-29', WeekCalendar::weekStart('2026-13-40', 1, '2026-07-05'));
+    }
+
+    public function testDaysReturnsSevenBucketsWithSlotsOnTheirDates(): void
+    {
+        $monday    = new AvailabilitySlot(5, '2026-07-06 09:00:00', '2026-07-06 10:00:00', 60, null, false, null, 1);
+        $mondayTwo = new AvailabilitySlot(5, '2026-07-06 10:00:00', '2026-07-06 11:00:00', 60, null, false, null, 2);
+        $thursday  = new AvailabilitySlot(5, '2026-07-09 14:00:00', '2026-07-09 15:00:00', 60, null, false, null, 3);
+        $nextWeek  = new AvailabilitySlot(5, '2026-07-13 09:00:00', '2026-07-13 10:00:00', 60, null, false, null, 4);
+
+        $days = WeekCalendar::days('2026-07-06', [$monday, $mondayTwo, $thursday, $nextWeek]);
+
+        self::assertCount(7, $days);
+        self::assertSame('2026-07-06', $days[0]['date']);
+        self::assertSame('2026-07-12', $days[6]['date']);
+
+        self::assertCount(2, $days[0]['slots']);
+        self::assertSame(3, $days[3]['slots'][0]->id);
+        self::assertSame([], $days[1]['slots']);
+
+        // A slot outside the week is not bucketed anywhere.
+        $ids = array_merge(...array_map(
+            static fn (array $day): array => array_map(static fn (AvailabilitySlot $s): ?int => $s->id, $day['slots']),
+            $days
+        ));
+        self::assertNotContains(4, $ids);
+    }
+
+    public function testDaysCrossesMonthBoundary(): void
+    {
+        $days = WeekCalendar::days('2026-06-29', []);
+
+        self::assertSame('2026-06-29', $days[0]['date']);
+        self::assertSame('2026-07-05', $days[6]['date']);
+    }
+}