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/App.tsx

@@ -120,7 +120,10 @@ export function Workspace() {
   const [exporting, setExporting] = useState(false);
   const [user, setUser] = useState<User | null>(null);
   const [saved, setSaved] = useState<SavedRecipe[]>([]);
-  const [authOpen, setAuthOpen] = useState(false);
+  // null = closed. The landing's register link is /app?auth=1, so it opens the
+  // dialog straight on the sign-up tab.
+  const [authMode, setAuthMode] = useState<'login' | 'signup' | null>(() =>
+    (new URLSearchParams(window.location.search).has('auth') ? 'signup' : null));
 
   const renderToken = useRef(0);
 
@@ -282,7 +285,7 @@ export function Workspace() {
 
   const saveRecipe = useCallback(async () => {
     if (!user) {
-      setAuthOpen(true);
+      setAuthMode('login');
       return;
     }
     const name = window.prompt(t('save.name'), recipe.name);
@@ -688,7 +691,7 @@ export function Workspace() {
         onReset={reset}
         onExport={exportPhoto}
         onSave={saveRecipe}
-        onAuth={() => setAuthOpen(true)}
+        onAuth={() => setAuthMode('login')}
         onLogout={async () => {
           await api.logout().catch(() => undefined);
           setUser(null);
@@ -791,11 +794,12 @@ export function Workspace() {
         />
       </div>
 
-      {authOpen ? (
+      {authMode ? (
         <AuthModal
-          onClose={() => setAuthOpen(false)}
+          initialMode={authMode}
+          onClose={() => setAuthMode(null)}
           onDone={async () => {
-            setAuthOpen(false);
+            setAuthMode(null);
             const me = await api.me().catch(() => null);
             if (me) setUser(me.user);
           }}