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/shared/primitives/DateInput.svelte

@@ -11,6 +11,11 @@ interface Props {
 	placeholder?: string;
 	class?: string;
 	onchange?: () => void;
+	ariaLabel?: string;
+	ariaDescribedby?: string;
+	// Escape hatch for callers that need the raw element, e.g. to set a custom
+	// validity and block native form submission while the date is partial.
+	inputEl?: HTMLInputElement;
 }
 
 let {
@@ -20,7 +25,10 @@ let {
 	id,
 	placeholder,
 	class: className = '',
-	onchange
+	onchange,
+	ariaLabel,
+	ariaDescribedby,
+	inputEl = $bindable()
 }: Props = $props();
 
 let display = $state(isoToGerman(value ?? ''));
@@ -76,10 +84,13 @@ function handleInput(e: Event) {
 </script>
 
 <input
+	bind:this={inputEl}
 	type="text"
 	inputmode="numeric"
 	maxlength="10"
 	id={id}
+	aria-label={ariaLabel}
+	aria-describedby={ariaDescribedby}
 	value={display}
 	placeholder={placeholder ?? m.form_placeholder_date()}
 	oninput={handleInput}