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/routes/persons/[id]/edit/PersonEditForm.svelte.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
-import { page } from 'vitest/browser';
+import { page, userEvent } from 'vitest/browser';
 import PersonEditForm from './PersonEditForm.svelte';
 
 afterEach(cleanup);
@@ -219,4 +219,16 @@ describe('PersonEditForm', () => {
 		const select = (await page.getByLabelText(/^generation$/i).element()) as HTMLSelectElement;
 		expect(select.value).toBe('');
 	});
+
+	// ─── partial-date guard (#812 review) ────────────────────────────────────────
+
+	it('blocks submission while a stored birth date is partially edited (no silent clear)', async () => {
+		render(PersonEditForm, { props: { person: personPersonal } });
+
+		await userEvent.fill(page.getByLabelText(/^geburtsdatum$/i), '14.03.');
+
+		const birthInput = (await page.getByLabelText(/^geburtsdatum$/i).element()) as HTMLInputElement;
+		expect(birthInput.checkValidity()).toBe(false);
+		await expect.element(page.getByText(/Bitte im Format TT\.MM\.JJJJ/)).toBeVisible();
+	});
 });