test(mocks): wire DocumentRow spec to shared __mocks__/$app/navigation

Smallest possible first step of issue #560. Adds a shared
__mocks__/$app/navigation.ts exporting every nav function as vi.fn(),
and switches one consumer (DocumentRow.svelte.spec.ts) from a per-spec
factory to vi.mock('\$app/navigation') (no factory) to verify Vitest's
__mocks__/ redirect resolves for SvelteKit virtual modules in
browser-mode tests.

CI is the gate: if the migrated spec passes alongside the 35 unchanged
factory call sites, the same redirect pattern is applied across the
remaining $app/navigation, $app/state, $app/forms and paraglide
factories.

Per ADR-012, eliminating factory mocks shrinks the birpc teardown-race
surface; this commit is the first concrete cut.

Refs #560.

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

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

@@ -0,0 +1,20 @@
+// Shared mock for SvelteKit's $app/navigation virtual module.
+// Activated by calling `vi.mock('$app/navigation')` (no factory) in a spec.
+// Per ADR-012: eliminating per-spec factory bodies removes 36 birpc-race surface
+// points; the unified mock keeps every nav export available as a vi.fn().
+//
+// IMPORTANT: consuming specs MUST call `vi.clearAllMocks()` (or per-mock
+// `mockClear()`) in `afterEach` — otherwise call counts leak between tests.
+import { vi } from 'vitest';
+
+export const goto = vi.fn(async () => {});
+export const invalidate = vi.fn(async () => {});
+export const invalidateAll = vi.fn(async () => {});
+export const beforeNavigate = vi.fn();
+export const afterNavigate = vi.fn();
+export const preloadCode = vi.fn(async () => {});
+export const preloadData = vi.fn(async () => {});
+export const pushState = vi.fn();
+export const replaceState = vi.fn();
+export const disableScrollHandling = vi.fn();
+export const onNavigate = vi.fn(() => () => {});

frontend/src/lib/document/DocumentRow.svelte.spec.ts

@@ -6,7 +6,7 @@ import DocumentRow from './DocumentRow.svelte';
 import { bulkSelectionStore } from '$lib/document/bulkSelection.svelte';
 import type { components } from '$lib/generated/api';
 
-vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
+vi.mock('$app/navigation');
 
 afterEach(() => {
 	cleanup();