Add personal Unix group member management

Every account gets a personal posixGroup at creation (its primary GID
holder) but there was no way to manage its memberUid list -- add
add/remove endpoints and a profile-page UI (admin-only), reusing the
userSelect widget already built for the manager field.

Bumps to v1.1.9.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KDEx8ghuZR61pqPXc6da9C

nodejs/package.json

@@ -1,6 +1,6 @@
 {
   "name": "t42-sso-manager",
-  "version": "1.1.8",
+  "version": "1.1.9",
   "private": true,
   "author": [
     {