test(hooks): migrate useUnsavedWarning spec to shared $app/navigation mock

Replaces the local beforeNavigate-capture plumbing and simulateNavigate
helper with the shared $mocks/$app/navigation module via a sync factory.
The per-test reset now comes from the shared module's embedded beforeEach.
Part of #560.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/hooks/useUnsavedWarning.svelte.test.ts

@@ -1,35 +1,12 @@
-import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
+import * as navMock from '$mocks/$app/navigation';
 
-// Capture the beforeNavigate callback so tests can simulate navigation events
-let registeredBeforeNavigate:
-	| ((nav: { cancel: () => void; to: { url: { href: string } } | null }) => void)
-	| null = null;
+vi.mock('$app/navigation', () => ({ ...navMock }));
 
-const mockGoto = vi.fn();
-
-vi.mock('$app/navigation', () => ({
-	beforeNavigate: vi.fn((fn: typeof registeredBeforeNavigate) => {
-		registeredBeforeNavigate = fn;
-	}),
-	goto: mockGoto
-}));
+const { simulateNavigate, goto: mockGoto } = navMock;
 
 const { createUnsavedWarning } = await import('./useUnsavedWarning.svelte');
 
-function simulateNavigate(href: string | null = '/somewhere') {
-	const cancel = vi.fn();
-	registeredBeforeNavigate?.({
-		cancel,
-		to: href ? { url: { href } } : null
-	});
-	return cancel;
-}
-
-beforeEach(() => {
-	registeredBeforeNavigate = null;
-	mockGoto.mockClear();
-});
-
 describe('createUnsavedWarning', () => {
 	it('isDirty starts false', () => {
 		const w = createUnsavedWarning();