feat: German as the main language, English under /en, with a language switch

The site now exists in German (at the root, with /impressum and /datenschutz)
and English (/en). A request without a language follows the visitor's choice
from the language switch, then the browser language, then German; the old
English legal addresses move to /en for good.

All visible texts are data in src/content/de.ts and en.ts, so components hold
none of their own. The header carries a DE/EN switch whose bit curtain covers
the page while it is swapped and reopens at the same reading position. Title,
description, canonical, hreflang, Open Graph, the share image, the sitemap and
the structured data are per language. The privacy notice now describes the
cookie that remembers the chosen language.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/components/ContentProvider.tsx

@@ -0,0 +1,34 @@
+"use client";
+
+import * as React from "react";
+import type { Content } from "@/src/content/types";
+import type { Locale } from "@/src/domain/locale";
+
+type Value = { readonly locale: Locale; readonly content: Content };
+
+const ContentContext = React.createContext<Value | null>(null);
+
+/**
+ * ContentProvider: hands the language of the page and its texts to the client
+ * components. The layout picks both; only the texts of that one language are
+ * sent to the browser.
+ */
+export const ContentProvider: React.FC<
+  Value & { children: React.ReactNode }
+> = ({ locale, content, children }) => {
+  const value = React.useMemo(() => ({ locale, content }), [locale, content]);
+  return (
+    <ContentContext.Provider value={value}>{children}</ContentContext.Provider>
+  );
+};
+
+const useValue = (): Value => {
+  const value = React.useContext(ContentContext);
+  if (value === null) {
+    throw new Error("useContent and useLocale need a ContentProvider above.");
+  }
+  return value;
+};
+
+export const useContent = (): Content => useValue().content;
+export const useLocale = (): Locale => useValue().locale;