fix(DateInput): fire onchange when field is cleared

Clearing the input set value='' but did not call onchange, so the
korrespondenz filter strip never re-fetched. Added onchange?.() in the
empty-display branch and added a test that confirms the callback fires.

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

frontend/src/lib/components/DateInput.svelte

@@ -31,6 +31,7 @@ function handleInput(e: Event) {
 	if (result.display === '') {
 		value = '';
 		errorMessage = null;
+		onchange?.();
 		return;
 	}
 

frontend/src/lib/components/DateInput.svelte.spec.ts

@@ -173,6 +173,14 @@ describe('DateInput – clearing the date', () => {
 		expect(value).toBe('');
 		expect(errorMessage).toBeNull();
 	});
+
+	it('fires onchange when the field is cleared', async () => {
+		let called = 0;
+		render(DateInput, { value: '2024-12-20', onchange: () => called++ });
+		const input = page.getByRole('textbox');
+		await input.fill('');
+		expect(called).toBeGreaterThan(0);
+	});
 });
 
 // ─── Hidden input ─────────────────────────────────────────────────────────────