test(mocks): add shared $app/navigation mock with simulateNavigate

Exports the standard nav functions as vi.fn() and a beforeNavigate that
captures the registered callback. The exported simulateNavigate(href)
helper fires that callback and returns the cancel spy — the whole
capture-and-fire pattern lives in the shared module, not the raw callback.
An embedded beforeEach clears the captured callback and the mock call
histories before every test. Part of #560.

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

frontend/src/__mocks__/$app/navigation.ts

@@ -0,0 +1,62 @@
+// Shared browser-test mock body for the SvelteKit `$app/navigation` virtual module.
+//
+// Imported into a sync vi.mock factory via the $mocks alias:
+//   import * as navMock from '$mocks/$app/navigation';
+//   vi.mock('$app/navigation', () => ({ ...navMock }));
+//
+// All navigation functions are vi.fn() stubs. `beforeNavigate` additionally
+// captures the registered callback so a test can drive it through the exported
+// `simulateNavigate(href)` helper — the whole capture-and-fire pattern lives
+// here, not the raw callback. The embedded `beforeEach` clears the captured
+// callback and the mock call histories before every test, so isolation is
+// structural. See ADR-012.
+
+import { beforeEach, vi } from 'vitest';
+
+type BeforeNavigateCallback = (nav: {
+	cancel: () => void;
+	to: { url: { href: string } } | null;
+}) => void;
+
+let _registeredBeforeNavigate: BeforeNavigateCallback | null = null;
+
+export const goto = vi.fn();
+export const invalidate = vi.fn();
+export const invalidateAll = vi.fn();
+export const beforeNavigate = vi.fn((fn: BeforeNavigateCallback) => {
+	_registeredBeforeNavigate = fn;
+});
+export const afterNavigate = vi.fn();
+export const preloadCode = vi.fn();
+export const preloadData = vi.fn();
+export const pushState = vi.fn();
+export const replaceState = vi.fn();
+export const disableScrollHandling = vi.fn();
+export const onNavigate = vi.fn();
+
+const _navMocks = [
+	goto,
+	invalidate,
+	invalidateAll,
+	beforeNavigate,
+	afterNavigate,
+	preloadCode,
+	preloadData,
+	pushState,
+	replaceState,
+	disableScrollHandling,
+	onNavigate
+];
+
+// Fire the captured beforeNavigate callback as if navigating to `href`.
+// Returns the cancel spy so the test can assert whether navigation was blocked.
+export function simulateNavigate(href: string | null = '/somewhere'): ReturnType<typeof vi.fn> {
+	const cancel = vi.fn();
+	_registeredBeforeNavigate?.({ cancel, to: href ? { url: { href } } : null });
+	return cancel;
+}
+
+beforeEach(() => {
+	_registeredBeforeNavigate = null;
+	_navMocks.forEach((mock) => mock.mockClear());
+});