Make WP admins instructors too, and add an Access toggle page

A WordPress administrator previously inherited the studio-admin
capabilities but not `manage_availability`, so the studio owner running
as an admin had no way to reach "My Availability" or act as the
instructor — breaking single-instructor businesses.

Grant the instructor capabilities to administrators as well (via the
existing `user_has_cap` filter), and make both grants — studio-admin and
instructor — independently toggleable from a new Access admin page.

- RoleManager: extract `INSTRUCTOR_CAPS`; apply studio and instructor
  cap sets to administrators, each gated on a stored toggle (default on).
- AccessSettings + templates/admin/access.php: two options
  (`us_admin_grant_studio` / `us_admin_grant_instructor`), gated on the
  core `manage_options` capability so disabling a grant can never lock an
  administrator out of re-enabling it.
- AdminMenu: register the Access page after Studio Settings; keep the
  studio sidebar separator visible for any administrator.
- Tests for the toggles and the new settings reader; docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/Unit/Auth/AccessSettingsTest.php

@@ -0,0 +1,34 @@
+<?php
+declare(strict_types=1);
+
+namespace Unsupervised\Schedular\Tests\Unit\Auth;
+
+use Brain\Monkey\Functions;
+use Unsupervised\Schedular\Auth\AccessSettings;
+use Unsupervised\Schedular\Tests\Unit\TestCase;
+
+class AccessSettingsTest extends TestCase
+{
+    public function testBothGrantsDefaultOnWhenUnset(): void
+    {
+        // get_option returns the supplied default when the option is unset.
+        Functions\when('get_option')->alias(static fn(string $name, $default) => $default);
+
+        $settings = new AccessSettings();
+
+        self::assertTrue($settings->adminsAreStudioAdmins());
+        self::assertTrue($settings->adminsAreInstructors());
+    }
+
+    public function testStoredZeroDisablesAGrant(): void
+    {
+        Functions\when('get_option')->alias(static function (string $name) {
+            return $name === AccessSettings::OPT_GRANT_STUDIO ? '0' : '1';
+        });
+
+        $settings = new AccessSettings();
+
+        self::assertFalse($settings->adminsAreStudioAdmins());
+        self::assertTrue($settings->adminsAreInstructors());
+    }
+}