fix: a calendar date is the same day everywhere; legal pages use the shared language picker

formatDate read a date-only value as UTC midnight and showed it in the viewer's zone, so the legal pages' last update read a day early west of UTC. Date-only values are shown in UTC. The legal pages' hand-rolled language buttons are the landing page's LanguageSwitcher. formatDateTime and formatDate get the two-language test formatNumber already had.

frontend/src/lib/format.ts

@@ -176,8 +176,16 @@ export function formatDateTime(iso: string | null | undefined): string {
 }
 
 // Absolute day-precision date (e.g. "12 Jan 2017"), shown alongside a relative time. Empty for null.
+// A DATE-only value ("2026-09-17": a release, the legal pages' last update) is a calendar day, not an
+// instant: `new Date` reads it as UTC midnight, which is still the day before west of UTC, so it is
+// shown in UTC. A timestamp is shown in the viewer's own zone, as it should be.
 export function formatDate(iso: string | null | undefined, lang: string): string {
-  return iso
-    ? new Date(iso).toLocaleDateString(lang, { year: "numeric", month: "short", day: "numeric" })
-    : "";
+  if (!iso) return "";
+  const dateOnly = /^\d{4}-\d{2}-\d{2}$/.test(iso);
+  return new Date(iso).toLocaleDateString(lang, {
+    year: "numeric",
+    month: "short",
+    day: "numeric",
+    ...(dateOnly ? { timeZone: "UTC" } : {}),
+  });
 }