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]>

src/Availability/WeekCalendar.php

@@ -49,6 +49,36 @@ class WeekCalendar {
 		return $days;
 	}
 
+	/**
+	 * Bucket arbitrary items into the seven days of the week starting at
+	 * `$weekStart` (`Y-m-d`), using `$dayOf` to extract each item's `Y-m-d` day.
+	 * Every day is present, empty or not, in calendar order.
+	 *
+	 * @template T
+	 * @param list<T> $items
+	 * @param callable(T): string $dayOf
+	 * @return list<array{date: string, items: list<T>}>
+	 */
+	public static function bucket( string $weekStart, array $items, callable $dayOf ): array {
+		$start = self::parseDay( $weekStart ) ?? new \DateTimeImmutable( 'today' );
+
+		$byDay = [];
+		foreach ( $items as $item ) {
+			$byDay[ $dayOf( $item ) ][] = $item;
+		}
+
+		$days = [];
+		for ( $i = 0; $i < 7; $i++ ) {
+			$date   = $start->modify( '+' . $i . ' days' )->format( 'Y-m-d' );
+			$days[] = [
+				'date'  => $date,
+				'items' => $byDay[ $date ] ?? [],
+			];
+		}
+
+		return $days;
+	}
+
 	private static function parseDay( string $value ): ?\DateTimeImmutable {
 		$day = \DateTimeImmutable::createFromFormat( '!Y-m-d', $value );