feat(planner): add UndoBar component with 4s auto-dismiss

Shows undo notification after slot add/replace. Rückgängig button
calls onundo, auto-dismisses after 4s via ondismiss callback.
Also patches test-setup for userEvent + fake timers compatibility.

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

frontend/src/lib/planner/UndoBar.test.ts

@@ -0,0 +1,56 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { render, screen, act } from '@testing-library/svelte';
+import userEvent from '@testing-library/user-event';
+import UndoBar from './UndoBar.svelte';
+
+describe('UndoBar', () => {
+	beforeEach(() => {
+		vi.useFakeTimers();
+	});
+
+	afterEach(() => {
+		vi.useRealTimers();
+	});
+
+	it('is not mounted when visible is false', () => {
+		render(UndoBar, { props: { visible: false, message: 'Test', onundo: vi.fn(), ondismiss: vi.fn() } });
+		expect(screen.queryByTestId('undo-bar')).toBeNull();
+	});
+
+	it('is mounted and shows message when visible is true', () => {
+		render(UndoBar, { props: { visible: true, message: 'Gericht hinzugefügt', onundo: vi.fn(), ondismiss: vi.fn() } });
+		expect(screen.getByTestId('undo-bar')).toBeTruthy();
+		expect(screen.getByText('Gericht hinzugefügt')).toBeTruthy();
+	});
+
+	it('shows Rückgängig button', () => {
+		render(UndoBar, { props: { visible: true, message: 'Test', onundo: vi.fn(), ondismiss: vi.fn() } });
+		expect(screen.getByRole('button', { name: /Rückgängig/i })).toBeTruthy();
+	});
+
+	it('calls onundo when Rückgängig is clicked', async () => {
+		const onundo = vi.fn();
+		render(UndoBar, { props: { visible: true, message: 'Test', onundo, ondismiss: vi.fn() } });
+		await userEvent.click(screen.getByRole('button', { name: /Rückgängig/i }));
+		expect(onundo).toHaveBeenCalledOnce();
+	});
+
+	it('calls ondismiss after 4 seconds', async () => {
+		const ondismiss = vi.fn();
+		render(UndoBar, { props: { visible: true, message: 'Test', onundo: vi.fn(), ondismiss } });
+		await act(() => { vi.advanceTimersByTime(4000); });
+		expect(ondismiss).toHaveBeenCalledOnce();
+	});
+
+	it('does not call ondismiss before 4 seconds', async () => {
+		const ondismiss = vi.fn();
+		render(UndoBar, { props: { visible: true, message: 'Test', onundo: vi.fn(), ondismiss } });
+		await act(() => { vi.advanceTimersByTime(3999); });
+		expect(ondismiss).not.toHaveBeenCalled();
+	});
+
+	it('has role="status" for accessibility', () => {
+		render(UndoBar, { props: { visible: true, message: 'Test', onundo: vi.fn(), ondismiss: vi.fn() } });
+		expect(screen.getByRole('status')).toBeTruthy();
+	});
+});