v4.0 — Authentik 2026.5.3 compatibility

This release fixes the graphical regressions introduced by recent Authentik updates and is tested against Authentik 2026.5.3.

Fixes

Login logo: shrunk the oversized logo (was rendering at 333px) and tightened the empty space above/below it so the login box is no longer unnecessarily tall.
User / applications background: fixed the background being cut off / showing a grey block. The Authentik update added a fixed background-wrapper overlay that covered the theme background — it is now made transparent so the background image and the blurred header show correctly.
Language selector:
- Repositioned so it sits comfortably inside the rounded login box instead of overflowing its edge.
- Fully themed dropdown (dark glassmorphism panel) using the modern customizable-select API (appearance: base-select / ::picker(select)), with a transparent option list and hover-only highlight.
- Fixed the icon intercepting clicks (dropdown now opens reliably) and pinned the popup width so it no longer shrinks on hover.

Language switching not translating the interface is an Authentik brand/config behaviour (forced default locale + custom flow strings), not a theme issue.
If you use Cloudflare with a proxied domain, purge the cache after updating the CSS.