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/AvailabilitySlotTest.php

@@ -84,6 +84,51 @@ class AvailabilitySlotTest extends TestCase
         self::assertNull(AvailabilitySlot::normalizeDateTime("2026-04-01 09:00:00'); DROP TABLE x;--"));
     }
 
+    public function testSplitByDurationChunksWindowIntoLessonLengthSlots(): void
+    {
+        $window = new AvailabilitySlot(5, '2026-07-06 09:00:00', '2026-07-06 12:00:00', 60, 8);
+
+        $slots = $window->splitByDuration();
+
+        self::assertCount(3, $slots);
+        self::assertSame('2026-07-06 09:00:00', $slots[0]->startDt);
+        self::assertSame('2026-07-06 10:00:00', $slots[0]->endDt);
+        self::assertSame('2026-07-06 11:00:00', $slots[2]->startDt);
+        self::assertSame('2026-07-06 12:00:00', $slots[2]->endDt);
+
+        // Each chunk keeps the window's instructor, duration, and offering.
+        self::assertSame(5, $slots[1]->instructorId);
+        self::assertSame(60, $slots[1]->durationMinutes);
+        self::assertSame(8, $slots[1]->offeringId);
+    }
+
+    public function testSplitByDurationDropsRemainderShorterThanALesson(): void
+    {
+        $window = new AvailabilitySlot(5, '2026-07-06 09:00:00', '2026-07-06 16:30:00', 60);
+
+        $slots = $window->splitByDuration();
+
+        self::assertCount(7, $slots);
+        self::assertSame('2026-07-06 16:00:00', $slots[6]->endDt);
+    }
+
+    public function testSplitByDurationReturnsEmptyWhenWindowTooShort(): void
+    {
+        $window = new AvailabilitySlot(5, '2026-07-06 09:00:00', '2026-07-06 09:45:00', 60);
+
+        self::assertSame([], $window->splitByDuration());
+    }
+
+    public function testSplitByDurationHandlesThirtyMinuteLessons(): void
+    {
+        $window = new AvailabilitySlot(5, '2026-07-06 09:00:00', '2026-07-06 10:30:00', 30);
+
+        $slots = $window->splitByDuration();
+
+        self::assertCount(3, $slots);
+        self::assertSame('2026-07-06 09:30:00', $slots[0]->endDt);
+    }
+
     public function testToArrayContainsExpectedKeys(): void
     {
         $slot = new AvailabilitySlot(1, '2026-04-01 09:00:00', '2026-04-01 10:00:00', 30, 8, false, null, 10);