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

tests/Unit/Availability/AvailabilityEndpointTest.php

@@ -8,6 +8,7 @@ use Mockery;
 use Unsupervised\Schedular\Availability\AvailabilityEndpoint;
 use Unsupervised\Schedular\Availability\AvailabilityRepository;
 use Unsupervised\Schedular\Availability\AvailabilitySlot;
+use Unsupervised\Schedular\Availability\WindowValidator;
 use Unsupervised\Schedular\Offering\OfferingRepository;
 use Unsupervised\Schedular\Tests\Unit\TestCase;
 
@@ -26,7 +27,7 @@ class AvailabilityEndpointTest extends TestCase
 
         $this->repository = Mockery::mock(AvailabilityRepository::class);
         $this->offerings  = Mockery::mock(OfferingRepository::class);
-        $this->endpoint   = new AvailabilityEndpoint($this->repository, $this->offerings);
+        $this->endpoint   = new AvailabilityEndpoint($this->repository, new WindowValidator($this->offerings));
     }
 
     public function testCreateRejectsWindowSpanningMultipleDays(): void