fix(person): block submit while a life-date input is partial

A partial date (e.g. "14.03.") left the hidden ISO input empty, so
saving the edit form silently cleared a stored date. PersonLifeDateField
now delegates to the shared DateInput primitive (inline format error,
calendar validation) and sets a custom validity while the error is
present, so the browser blocks native submission for both person forms.
A full clear stays submittable - that is the intentional clear path.

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

frontend/src/lib/person/PersonLifeDateField.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import { onMount } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
-import { isoToGerman, handleGermanDateInput } from '$lib/shared/utils/date';
+import DateInput from '$lib/shared/primitives/DateInput.svelte';
 import type { DatePrecision } from '$lib/shared/utils/documentDate';
 
 // Only DAY / MONTH / YEAR are offered for life dates: RANGE and SEASON make no
@@ -26,8 +26,9 @@ let {
 	initialPrecision?: string | null;
 } = $props();
 
-let display = $state('');
 let iso = $state('');
+let errorMessage = $state<string | null>(null);
+let inputEl = $state<HTMLInputElement | undefined>();
 let precision = $state<DatePrecision>('DAY');
 
 // Seed once at mount (WhoWhenSection pattern): a later load() rerun must not
@@ -35,7 +36,6 @@ let precision = $state<DatePrecision>('DAY');
 onMount(() => {
 	if (initialIso) {
 		iso = initialIso;
-		display = isoToGerman(initialIso);
 	}
 	const offered = PERSON_DATE_PRECISIONS.some((p) => p.value === initialPrecision);
 	if (offered) {
@@ -47,11 +47,11 @@ onMount(() => {
 	}
 });
 
-function handleInput(e: Event) {
-	const result = handleGermanDateInput(e);
-	display = result.display;
-	iso = result.iso;
-}
+// A partial date leaves the hidden ISO empty — submitting then would silently
+// clear a stored date. Block native submission until completed or fully emptied.
+$effect(() => {
+	inputEl?.setCustomValidity(errorMessage ?? '');
+});
 
 const controlCls =
 	'block min-h-[44px] w-full rounded border border-line px-3 py-2 font-serif text-ink focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring';
@@ -63,18 +63,20 @@ const controlCls =
 	</legend>
 	<div class="flex flex-col gap-2 sm:flex-row">
 		<div class="flex-1">
-			<input
+			<DateInput
+				bind:value={iso}
+				bind:errorMessage={errorMessage}
+				bind:inputEl={inputEl}
+				name={name}
 				id={name}
-				type="text"
-				inputmode="numeric"
 				placeholder="TT.MM.JJJJ"
-				maxlength="10"
-				aria-label={legend}
-				value={display}
-				oninput={handleInput}
+				ariaLabel={legend}
+				ariaDescribedby={errorMessage ? `${name}-error` : undefined}
 				class={controlCls}
 			/>
-			<input type="hidden" name={name} value={iso} />
+			{#if errorMessage}
+				<p id="{name}-error" class="mt-1 font-sans text-xs text-red-600">{errorMessage}</p>
+			{/if}
 		</div>
 		<div class="flex-1">
 			<select