fix(20): WR-05 add aria-describedby to admin toggle for last-admin error

Link the admin role="switch" toggle button to the profile-error region
via aria-describedby so screen-reader users who activate the toggle
hear the last-admin error announcement — matching the existing link on
the display-name input.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/pwa/src/components/MemberEditorSheet.tsx

@@ -540,6 +540,7 @@ export function MemberEditorSheet({
                   role="switch"
                   aria-checked={isAdmin}
                   aria-label="Admin"
+                  aria-describedby={profileError ? 'profile-error' : undefined}
                   onClick={() => {
                     setProfileError(null);
                     setIsAdmin((prev) => !prev);