chore: merge origin/main into feat/issue-162-korrespondenz-redesign

Resolved conflicts:
- messages/de|en|es.json: kept all keys from both sides
- DateInput.svelte: kept HEAD API (onchange, not oninput/...rest) to match
  CorrespondenzFilterControls caller; incorporated main's isCalendarValid helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/DateInput.svelte

@@ -24,6 +24,16 @@ let {
 
 let display = $state(isoToGerman(value ?? ''));
 
+// ─── Validation helper ────────────────────────────────────────────────────
+function isCalendarValid(iso: string): boolean {
+	if (!iso) return false;
+	const [, mm, dd] = iso.match(/^\d{4}-(\d{2})-(\d{2})$/) ?? [];
+	const month = parseInt(mm, 10);
+	const day = parseInt(dd, 10);
+	return month >= 1 && month <= 12 && day >= 1 && day <= 31;
+}
+
+// ─── Input handler ────────────────────────────────────────────────────────
 function handleInput(e: Event) {
 	const result = handleGermanDateInput(e);
 	display = result.display;
@@ -42,14 +52,7 @@ function handleInput(e: Event) {
 	}
 
 	const iso = germanToIso(result.display);
-	if (!iso) {
-		value = '';
-		errorMessage = m.form_date_error();
-		return;
-	}
-
-	const [, mo, d] = iso.split('-').map(Number);
-	if (mo < 1 || mo > 12 || d < 1 || d > 31) {
+	if (!iso || !isCalendarValid(iso)) {
 		value = '';
 		errorMessage = m.form_date_error();
 		return;