revert(test): abandon shared-mock dedup — infeasible in vitest browser mode

CI proved cross-file sharing of a virtual-module mock body cannot work in
@vitest/browser-playwright 4.1.6: the static-import spread fails the hoist
("no top level variables"), and the await-vi.hoisted-import form fails to
parse ("Unexpected identifier 'vi'"). vi.hoisted has the same hoist
constraint as vi.mock, so there is no way to thread an external module's
body into the factory here.

Reverts Phase 1: restores the 4 $app/forms/$app/navigation specs to their
inline factories, inlines NotificationBell.spec's forms stub, deletes the
src/__mocks__/$app/* modules and the $mocks alias (vite, vitest-coverage,
kit). The no-factory-ban meta-test stays (no-factory vi.mock is still
banned). ADR-012 amended to record the infeasibility. Everything else
($app/state migration, confirm context-inject, notification refactor, the
pin, the meta-test) is unaffected. Part of #560.

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

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

@@ -1,12 +1,35 @@
-import { describe, it, expect, vi } from 'vitest';
-const navMock = await vi.hoisted(() => import('$mocks/$app/navigation'));
+import { describe, it, expect, vi, beforeEach } from 'vitest';
 
-vi.mock('$app/navigation', () => ({ ...navMock }));
+// Capture the beforeNavigate callback so tests can simulate navigation events
+let registeredBeforeNavigate:
+	| ((nav: { cancel: () => void; to: { url: { href: string } } | null }) => void)
+	| null = null;
 
-const { simulateNavigate, goto: mockGoto } = navMock;
+const mockGoto = vi.fn();
+
+vi.mock('$app/navigation', () => ({
+	beforeNavigate: vi.fn((fn: typeof registeredBeforeNavigate) => {
+		registeredBeforeNavigate = fn;
+	}),
+	goto: mockGoto
+}));
 
 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();