Stop the availability form failing in silence

Adding availability for 5:30-6:00 PM with the lesson length left on its
60-minute default saved nothing and said nothing. A window is stored as
consecutive lesson-length slots, so one that fits no lesson splits into
none: splitByDuration() returned [], createFromWindow() inserted
nothing, and addSlot() discarded the result and re-rendered the page
unchanged.

The REST endpoint already rejected that window with a 400. The admin
form checked the same rules separately, and its copy was both laxer and
mute — an unreadable date, an end before the start, and a two-day window
were bare `return`s, and it never checked offering ownership at all, so
a crafted POST could tie a slot to another instructor's offering and
inherit their price and payment routing.

Both callers now go through WindowValidator, which returns the window or
a WP_Error explaining the refusal. The endpoint returns that error as
is; the page renders its message as a notice. handleFormAction returns
a [notice, error] pair so deletes report themselves too, and a
successful add says how many slots it created.

Two failures could also go unnoticed underneath: wpdb::insert's result
was ignored, and insert_id still holds the previous statement's id after
a failed write, so a failure looked like a success — and could become
the recurrence group of a weekly series, orphaning every later
occurrence. weeks was unbounded server-side despite the form's max=52.

availability-admin.js narrows the lesson-length choices to those that
fit the window and blocks submission when none do, which is what makes
the original mistake hard to repeat. It is a convenience: the server
validates regardless.

Closes #130

src/Availability/AvailabilitySlot.php

@@ -7,6 +7,23 @@ use Unsupervised\Schedular\Val;
 
 class AvailabilitySlot {
 
+	/** Lesson length used when none was submitted. */
+	public const DEFAULT_DURATION_MINUTES = 60;
+
+	/**
+	 * Lesson lengths a window can be split into, offered by the availability
+	 * form. The form hides the ones a given window is too short for.
+	 *
+	 * @var list<int>
+	 */
+	public const DURATION_CHOICES = [ 30, 60 ];
+
+	/**
+	 * Ceiling on a weekly series, matching the form's `max`. Enforced in the
+	 * repository too, so a hand-crafted POST cannot ask for ten thousand rows.
+	 */
+	public const MAX_WEEKLY_OCCURRENCES = 52;
+
 	public function __construct(
 		public readonly int $instructorId,
 		public readonly string $startDt,