Give the landing the app's theme and language switches

The blueprint page was dark-only and English-only. It now carries the same
two controls the workspace TopBar has, in the nav's top-right cluster: ◐ flips
light/dark (a paper palette for the same funnel — surfaces and ink flip, the
amber/red accents stay) and VI/EN flips the language, with the whole page of
copy, the FAQ, the pricing tables and the VIP badge all following it. Amber
text switches to a darker #a16207 on the light theme so it keeps ~4.9:1 on
white.

The register account is back as the old landing had it: a "ĐĂNG KÝ" button
pointing at /app?auth=1, and that URL now actually opens the auth dialog on
its sign-up tab (AuthModal takes an initialMode). The nav also collapses to
the hamburger below 1160px now, and the logo/tools shrink below 680px, so the
row still fits a 360px phone.

docker/frontend/src/ui/AuthModal.tsx

@@ -6,14 +6,16 @@ import { api } from '../api';
 // just drives it. The demo is always one click away — the app is usable signed
 // out, and only export resolution / saving are gated (see config/tiers.ts).
 export function AuthModal({
+  initialMode = 'login',
   onClose,
   onDone,
 }: {
+  initialMode?: 'login' | 'signup';
   onClose: () => void;
   onDone: () => void;
 }) {
   const { t } = useI18n();
-  const [mode, setMode] = useState<'login' | 'signup'>('login');
+  const [mode, setMode] = useState<'login' | 'signup'>(initialMode);
   const [email, setEmail] = useState('');
   const [password, setPassword] = useState('');
   const [busy, setBusy] = useState(false);