deps: bump usher to 20260630-843b934 (themeable auth pages)

Picks up the shared render-auth-page shell + :brand/:head-html/:body-class
config hooks and stable usher-* class names, so Cave can style the OIDC
sign-in/TOTP/reset/verify pages. Wiring + usher-auth.css follow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>