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/test-setup.ts

@@ -1 +1,35 @@
 import '@testing-library/jest-dom/vitest';
+// Import @testing-library/svelte here so its beforeEach (setup/asyncWrapper=act)
+// is registered before our own beforeEach below.
+import '@testing-library/svelte';
+import { configure } from '@testing-library/dom';
+import { vi } from 'vitest';
+import { tick } from 'svelte';
+import userEvent from '@testing-library/user-event';
+
+// Patch userEvent direct-API methods to use delay:null when fake timers are
+// active. With delay:null, user-event's internal wait() short-circuits
+// (typeof null !== 'number') and no setTimeout is scheduled — so clicks and
+// other interactions work correctly under vi.useFakeTimers().
+const originalClick = userEvent.click.bind(userEvent);
+// @ts-expect-error patching direct API
+userEvent.click = (element: Element, options = {}) => {
+	if (vi.isFakeTimers()) {
+		// @ts-expect-error delay:null is a valid user-event option
+		return originalClick(element, { delay: null, ...options });
+	}
+	return originalClick(element, options);
+};
+
+// Also update asyncWrapper to call tick() after async operations so Svelte
+// DOM updates are flushed. @testing-library/svelte's act() already does this,
+// but we re-configure after it to preserve our fake-timer behaviour.
+beforeEach(() => {
+	configure({
+		asyncWrapper: async (fn: () => Promise<unknown>) => {
+			const result = await fn();
+			await tick();
+			return result;
+		}
+	});
+});