Fix field-length saves, student wp-admin access, and empty instructor picker

Three bug fixes for the 1.2.1 section:

- Fixed-size fields (question labels, offering titles/notes/e-transfer
  email, policy titles/slugs) no longer silently fail to save when the
  value exceeds its column length. The REST endpoints reject over-long
  values with a 400, the admin controllers refuse to insert them, and the
  form inputs carry a maxlength so the browser blocks over-long entry.
  Limits are MAX_* constants on the value objects, kept in lockstep with
  the schema columns.

- Students are kept out of wp-admin entirely. New StudentAdminGuard
  redirects front-end-only users (no back-office capability) away from the
  dashboard and hides the admin bar for them, while administrators, studio
  admins, and instructors keep full access.

- The Add/Edit Offering instructor picker now includes WordPress
  administrators when they act as instructors (the default single-account
  setup), so a solo studio owner is selectable instead of the dropdown
  being empty.

composer test (618), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/Unit/Offering/OfferingEndpointTest.php

@@ -118,4 +118,37 @@ class OfferingEndpointTest extends TestCase
 
         self::assertArrayNotHasKey('etransfer_email', $data[0]);
     }
+
+    public function testCreateRejectsTitleLongerThanColumnLimit(): void
+    {
+        Functions\when('sanitize_text_field')->returnArg();
+        Functions\when('sanitize_email')->returnArg();
+        $this->repository->shouldNotReceive('insert');
+
+        $request  = new \WP_REST_Request([
+            'kind'  => Offering::KIND_GROUP_CLASS,
+            'title' => str_repeat('a', Offering::MAX_TITLE_LENGTH + 1),
+        ]);
+        $response = $this->endpoint->create($request);
+
+        self::assertInstanceOf(\WP_Error::class, $response);
+        self::assertSame(400, $response->error_data['invalid_offering']['status']);
+    }
+
+    public function testCreateRejectsScheduleNoteLongerThanColumnLimit(): void
+    {
+        Functions\when('sanitize_text_field')->returnArg();
+        Functions\when('sanitize_email')->returnArg();
+        $this->repository->shouldNotReceive('insert');
+
+        $request  = new \WP_REST_Request([
+            'kind'          => Offering::KIND_GROUP_CLASS,
+            'title'         => 'Choir',
+            'schedule_note' => str_repeat('a', Offering::MAX_SCHEDULE_NOTE_LENGTH + 1),
+        ]);
+        $response = $this->endpoint->create($request);
+
+        self::assertInstanceOf(\WP_Error::class, $response);
+        self::assertSame(400, $response->error_data['invalid_offering']['status']);
+    }
 }