fix(admin): clear unsaved-changes guard before redirect on groups/new

Use createUnsavedWarning() + UnsavedWarningBanner to replace the inline
beforeNavigate/isDirty pattern, and add an enhance callback that calls
clearOnSuccess() before update() so the guard is disarmed before
SvelteKit's internal goto() fires on a redirect result.

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

frontend/src/routes/admin/groups/new/page.svelte.spec.ts

@@ -0,0 +1,124 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import Page from './+page.svelte';
+
+const enhanceCaptureRef = vi.hoisted(() => ({ submitFn: undefined as unknown }));
+
+vi.mock('$app/forms', () => ({
+	enhance: (_el: HTMLFormElement, fn?: unknown) => {
+		enhanceCaptureRef.submitFn = fn;
+		return { destroy: vi.fn() };
+	}
+}));
+vi.mock('$app/navigation', () => ({ beforeNavigate: vi.fn(), goto: vi.fn() }));
+
+import { beforeNavigate, goto } from '$app/navigation';
+
+afterEach(cleanup);
+
+// ─── Unsaved-changes guard ────────────────────────────────────────────────────
+
+describe('Admin new group page – unsaved-changes guard', () => {
+	beforeEach(() => {
+		vi.clearAllMocks();
+		enhanceCaptureRef.submitFn = undefined;
+	});
+
+	it('does not show unsaved warning initially', async () => {
+		render(Page, { props: { form: null } });
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).not.toBeInTheDocument();
+	});
+
+	it('cancels navigation and shows banner when form is dirty', async () => {
+		render(Page, { props: { form: null } });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/groups') } });
+
+		expect(cancel).toHaveBeenCalled();
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).toBeInTheDocument();
+	});
+
+	it('does not cancel navigation when form is clean', async () => {
+		render(Page, { props: { form: null } });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		const cancel = vi.fn();
+		callback({ cancel, to: { url: new URL('http://localhost/admin/groups') } });
+
+		expect(cancel).not.toHaveBeenCalled();
+	});
+
+	it('discard button calls goto with the target URL', async () => {
+		render(Page, { props: { form: null } });
+		const [callback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		callback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/groups') } });
+
+		await page.getByRole('button', { name: /verwerfen/i }).click();
+
+		expect(vi.mocked(goto)).toHaveBeenCalledWith('http://localhost/admin/groups');
+	});
+
+	it('clears banner when enhance callback receives a redirect result', async () => {
+		render(Page, { props: { form: null } });
+		const [navCallback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		navCallback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/groups') } });
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).toBeInTheDocument();
+
+		type SubmitFn = () => Promise<
+			(opts: { result: { type: string }; update: () => Promise<void> }) => Promise<void>
+		>;
+		const innerFn = await (enhanceCaptureRef.submitFn as SubmitFn)();
+		await innerFn({
+			result: { type: 'redirect', location: '/admin/groups', status: 303 },
+			update: vi.fn().mockResolvedValue(undefined)
+		});
+
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).not.toBeInTheDocument();
+
+		const cancel = vi.fn();
+		navCallback({ cancel, to: { url: new URL('http://localhost/admin/groups') } });
+		expect(cancel).not.toHaveBeenCalled();
+	});
+
+	it('keeps banner when enhance callback receives a failure result', async () => {
+		render(Page, { props: { form: null } });
+		const [navCallback] = vi.mocked(beforeNavigate).mock.calls[0];
+
+		document
+			.querySelector<HTMLInputElement>('input[name="name"]')!
+			.dispatchEvent(new InputEvent('input', { bubbles: true }));
+
+		navCallback({ cancel: vi.fn(), to: { url: new URL('http://localhost/admin/groups') } });
+		await expect.element(page.getByText(/ungespeicherte Änderungen/i)).toBeInTheDocument();
+
+		type SubmitFn = () => Promise<
+			(opts: { result: { type: string }; update: () => Promise<void> }) => Promise<void>
+		>;
+		const innerFn = await (enhanceCaptureRef.submitFn as SubmitFn)();
+		await innerFn({
+			result: { type: 'failure', status: 400, data: { error: 'Name bereits vergeben' } },
+			update: vi.fn().mockResolvedValue(undefined)
+		});
+
+		const cancel = vi.fn();
+		navCallback({ cancel, to: { url: new URL('http://localhost/admin/groups') } });
+		expect(cancel).toHaveBeenCalled();
+	});
+});