feat(website): persist language choice in a cookie

useLang now seeds from a first-party `dezky-lang` cookie on SSR init and
toggleLang writes it (12 months, SameSite=Lax). The chosen language now
survives reloads/return visits (fixes the reset-to-Danish quirk) and makes
the cookie-policy entry accurate.

apps/website/composables/useLanding.ts

@@ -6,7 +6,18 @@ import { makeTheme } from '~/utils/landingTokens'
 // translated). `dark` is kept as machinery from the design's Tweaks panel but
 // defaults to light — the primary theme the user landed on — and no toggle is
 // surfaced. Flip the default (or add a control) to enable dark later.
-export const useLang = () => useState<Lang>('dz-lang', () => 'da')
+
+// Language choice persists in the first-party `dezky-lang` cookie (see the
+// cookie policy). useState stays the reactive source of truth so a toggle in
+// one component updates the whole page live; we seed it from the cookie on SSR
+// init (so a reload keeps the chosen language) and write the cookie on change.
+const LANG_COOKIE = 'dezky-lang'
+const LANG_MAX_AGE = 60 * 60 * 24 * 365 // 12 months
+
+export const useLang = () => useState<Lang>('dz-lang', () => {
+  const saved = useCookie<Lang>(LANG_COOKIE).value
+  return saved === 'en' ? 'en' : 'da'
+})
 export const useDark = () => useState<boolean>('dz-dark', () => false)
 
 export const useTheme = () => {
@@ -21,7 +32,10 @@ export const useCopy = () => {
 
 export function toggleLang() {
   const lang = useLang()
-  lang.value = lang.value === 'da' ? 'en' : 'da'
+  const next: Lang = lang.value === 'da' ? 'en' : 'da'
+  lang.value = next
+  // Persist so the choice survives a reload / return visit.
+  useCookie<Lang>(LANG_COOKIE, { maxAge: LANG_MAX_AGE, sameSite: 'lax', path: '/' }).value = next
 }
 
 // Smooth-scroll to an in-page anchor, accounting for the sticky 72px nav.