test(mocks): switch $app/navigation factory mocks to shared __mocks__ stub

Migrates 36 of the remaining 37 vi.mock('$app/navigation', factory) call
sites in frontend/src/**/*.svelte.{spec,test}.ts to
vi.mock('$app/navigation') (no factory), so they all resolve to the
shared src/__mocks__/$app/navigation.ts stub introduced in commit
aea37250.

Special handling:
- DropZone.svelte.spec.ts: removed vi.hoisted invalidateAllMock; test
  body now imports invalidateAll from $app/navigation and uses
  vi.mocked(invalidateAll) for assertions.
- documents/bulk-edit/page.svelte.test.ts: removed the module-scope
  const gotoSpy = vi.fn() that was injected into the factory; test
  body now imports goto from $app/navigation and uses
  vi.mocked(goto) for assertions and mockClear().

Deferred: src/lib/shared/hooks/useUnsavedWarning.svelte.test.ts uses a
non-trivial beforeNavigate wrapper that captures the callback into
module-scope state (registeredBeforeNavigate) so tests can simulate
navigation events. That hybrid factory cannot be replaced with the
plain-vi.fn() shared stub without redesigning the test; it is left in
place and will be addressed separately.

Per ADR-012: eliminating factory bodies removes 36 birpc teardown-race
surface points. The shared mock keeps every nav export as a vi.fn().

Refs #560.

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

frontend/src/routes/briefwechsel/CorrespondenzHero.svelte.spec.ts

@@ -3,7 +3,7 @@ import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 import CorrespondenzHero from './CorrespondenzHero.svelte';
 
-vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
+vi.mock('$app/navigation');
 
 afterEach(cleanup);
 

frontend/src/routes/briefwechsel/page.svelte.spec.ts

@@ -3,7 +3,7 @@ import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 import Page from './+page.svelte';
 
-vi.mock('$app/navigation', () => ({ goto: vi.fn() }));
+vi.mock('$app/navigation');
 
 afterEach(cleanup);
 

frontend/src/routes/briefwechsel/page.svelte.test.ts

@@ -2,19 +2,7 @@ import { describe, it, expect, vi, afterEach } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page } from 'vitest/browser';
 
-vi.mock('$app/navigation', () => ({
-	beforeNavigate: () => {},
-	afterNavigate: () => {},
-	goto: vi.fn(),
-	invalidate: vi.fn(),
-	invalidateAll: vi.fn(),
-	preloadCode: vi.fn(),
-	preloadData: vi.fn(),
-	pushState: vi.fn(),
-	replaceState: vi.fn(),
-	disableScrollHandling: vi.fn(),
-	onNavigate: () => () => {}
-}));
+vi.mock('$app/navigation');
 
 const { default: BriefwechselPage } = await import('./+page.svelte');