Integrate Vue webclient into server: serve SPA from /, remove proxy.php, update auth redirects

Add missing text_msgs aliases and dev proxy for auth endpoints ...

- text-msgs.php: add unauthenticated, permission_denied, invalid_callback,
  auth_failed, session_expired error messages
- vite.config.js: add /auth proxy to PHP backend for dev server

Phase 6: Rename webclient-vue to webclient, match legacy dist structure