refactor(stammbaum): StammbaumSidePanel composes AddRelationshipForm — removes inline form duplication

Replaces the 86-line duplicated inline add-relationship form with
<AddRelationshipForm onSubmit={handleAddRelationship}>. The {#key node.id}
wrapper resets the form's open state when the selected tree node changes.
Year inputs now have <label> elements (WCAG 1.3.1) via the shared component.

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

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

@@ -4,6 +4,7 @@ import { page } from 'vitest/browser';
 import StammbaumSidePanel from './StammbaumSidePanel.svelte';
 
 vi.mock('$app/navigation', () => ({ invalidateAll: vi.fn() }));
+vi.mock('$app/forms', () => ({ enhance: () => () => {} }));
 vi.mock('$lib/components/PersonTypeahead.svelte', () => ({ default: () => null }));
 
 const makeNode = () => ({
@@ -54,6 +55,23 @@ describe('StammbaumSidePanel', () => {
 		await expect.element(page.getByText('Noch keine Beziehungen bekannt.')).toBeInTheDocument();
 	});
 
+	it('year inputs inside the add form have label elements (canWrite=true)', async () => {
+		render(StammbaumSidePanel, { node: makeNode(), onClose: vi.fn(), canWrite: true });
+		await expect.element(page.getByText('Noch keine Beziehungen bekannt.')).toBeInTheDocument();
+		const addBtn = [...document.querySelectorAll<HTMLButtonElement>('button')].find((b) =>
+			/Beziehung hinzufügen/i.test(b.textContent ?? '')
+		);
+		addBtn!.click();
+		await expect.element(page.getByRole('combobox')).toBeInTheDocument();
+		const yearInputs = [...document.querySelectorAll('input')].filter(
+			(i) => i.inputMode === 'numeric'
+		);
+		expect(yearInputs.length).toBeGreaterThan(0);
+		for (const input of yearInputs) {
+			expect(input.closest('label')).not.toBeNull();
+		}
+	});
+
 	it('shows loading indicator while fetching', async () => {
 		let resolveFirst: (v: unknown) => void;
 		vi.stubGlobal(