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>

docs/features/user-roles.md

@@ -16,11 +16,22 @@ Runs the studio. Logs in via standard wp-admin. Can:
 **Capabilities:** `read`, `manage_instructors`, `manage_offerings`, `manage_questions`, `manage_policies`, `manage_billing`, `view_all_lessons`, `view_all_payments`, `export_payments`
 
 > Any WordPress **administrator** (`manage_options`) implicitly holds every
-> studio-admin capability above, so the site owner runs the studio without being
-> assigned the `us_studio_admin` role. This is applied dynamically via the
-> `user_has_cap` filter (`RoleManager::grantStudioCapsToAdministrators()`) — it
-> persists nothing and is removed when the plugin is deactivated. The
-> `us_studio_admin` role exists for non-administrator staff who manage the studio.
+> studio-admin capability above **and the instructor capabilities** (notably
+> `manage_availability`), so a single-instructor studio owner can run the business
+> and teach — managing their own availability and lessons — from one account,
+> without being assigned the `us_studio_admin` or `us_instructor` role. This is
+> applied dynamically via the `user_has_cap` filter
+> (`RoleManager::grantStudioCapsToAdministrators()`) — it persists nothing and is
+> removed when the plugin is deactivated. The `us_studio_admin` role exists for
+> non-administrator staff who manage the studio.
+>
+> Both grants can be turned off independently on the **Access** admin page
+> (`AccessSettings`, options `us_admin_grant_studio` / `us_admin_grant_instructor`,
+> both default on) — for example, when dedicated `us_studio_admin` or
+> `us_instructor` accounts run the studio and administrators should not appear as
+> studio staff. That page is gated on the core `manage_options` capability (which
+> the plugin never grants or revokes), so an administrator can always reach it to
+> re-enable a grant; disabling one can never lock them out.
 
 ### Instructor (`us_instructor`)
 Created by the studio admin. Logs in via standard wp-admin. Can: