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/domain/template.ts

@@ -0,0 +1,13 @@
+/**
+ * Texts with named blanks, such as "{name} on a phone". The page texts are plain
+ * data in two languages, so a sentence with a variable part keeps a blank for it.
+ */
+export type TemplateValues = Readonly<Record<string, string | number>>;
+
+const BLANK = /\{(\w+)\}/g;
+
+export function fill(template: string, values: TemplateValues): string {
+  return template.replace(BLANK, (blank, name: string) =>
+    Object.hasOwn(values, name) ? String(values[name]) : blank,
+  );
+}