Default lessons to a week view on the booking page and in wp-admin

The front-end booking calendar now opens in the Week view (anchored to the
week of the earliest open slot) with List still available. The Scheduler and
My Lessons admin pages gain a week calendar (usc_view/usc_week, bucketed via
a new generic WeekCalendar::bucket()) and open in it by default; the original
table remains as the List view since it carries the HST / e-transfer forms.

Closes #76

Co-Authored-By: Claude Fable 5 <[email protected]>

tests/Unit/Availability/WeekCalendarTest.php

@@ -63,4 +63,30 @@ class WeekCalendarTest extends TestCase
         self::assertSame('2026-06-29', $days[0]['date']);
         self::assertSame('2026-07-05', $days[6]['date']);
     }
+
+    public function testBucketGroupsItemsByExtractedDay(): void
+    {
+        $items = [
+            ['day' => '2026-07-06', 'label' => 'a'],
+            ['day' => '2026-07-06', 'label' => 'b'],
+            ['day' => '2026-07-09', 'label' => 'c'],
+            ['day' => '2026-07-13', 'label' => 'outside'],
+            ['day' => '', 'label' => 'dayless'],
+        ];
+
+        $days = WeekCalendar::bucket('2026-07-06', $items, static fn (array $i): string => $i['day']);
+
+        self::assertCount(7, $days);
+        self::assertSame('2026-07-06', $days[0]['date']);
+        self::assertSame('2026-07-12', $days[6]['date']);
+
+        self::assertSame(['a', 'b'], array_column($days[0]['items'], 'label'));
+        self::assertSame(['c'], array_column($days[3]['items'], 'label'));
+        self::assertSame([], $days[1]['items']);
+
+        // Items outside the week (or with no day) are not bucketed anywhere.
+        $labels = array_merge(...array_column($days, 'items'));
+        self::assertNotContains('outside', array_column($labels, 'label'));
+        self::assertNotContains('dayless', array_column($labels, 'label'));
+    }
 }