fix: vitrine/marketing pages use visitor's browser locale, not always English (v0.78.0)

getMarketingLocale() only ever read the marketing_locale cookie, defaulting anonymous first-time visitors to English until they manually used the language switcher — even though the switcher and a full French translation already existed. Now falls back to parsing Accept-Language when no cookie is set; an explicit switcher choice still wins on every later visit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/marketing-locale.ts

@@ -12,10 +12,31 @@ export { MARKETING_LOCALE_COOKIE };
  * which persists to `users.locale` via an authenticated PATCH that would
  * just 401 for an anonymous visitor and revert the UI.
  */
+/** Parses an `Accept-Language` header and returns the first supported
+ * locale it lists, in the browser's stated preference order — ignoring
+ * unsupported languages (e.g. "de", "es") rather than stopping at them. */
+function localeFromAcceptLanguage(header: string | null): Locale | null {
+  if (!header) return null;
+  const tags = header
+    .split(",")
+    .map((part) => part.split(";")[0]!.trim().toLowerCase())
+    .filter(Boolean);
+  for (const tag of tags) {
+    if (tag.startsWith("fr")) return "fr";
+    if (tag.startsWith("en")) return "en";
+  }
+  return null;
+}
+
 export async function getMarketingLocale(): Promise<Locale> {
   const store = await cookies();
-  const value = store.get(MARKETING_LOCALE_COOKIE)?.value;
-  return value === "fr" ? "fr" : "en";
+  const cookieValue = store.get(MARKETING_LOCALE_COOKIE)?.value;
+  if (cookieValue === "fr" || cookieValue === "en") return cookieValue;
+
+  // No explicit choice saved yet (first visit, or cookies cleared) — honor
+  // the visitor's browser language instead of always defaulting to English.
+  const headerStore = await headers();
+  return localeFromAcceptLanguage(headerStore.get("accept-language")) ?? "en";
 }
 
 /** A logged-in visitor's own saved locale wins (matches what they see