test(web): stub SvelteKit app modules suite-wide so no test loads the client runtime (#3943)

The home page reaches the real $app/navigation through AlbumCard and
AlbumMenu. That loads SvelteKit's client runtime, whose $app/paths reads
__SVELTEKIT_PAYLOAD__ at module load. The global is only there when the
kit plugin's define reaches the module, and under vitest that is not
reliable: page.test.ts failed to load on CI run 6576 and passed on its
re-run. #374 was the same class of failure.

vitest.setup.ts now mocks $app/navigation, $app/state and $app/paths for
every test. Per-file mocks still win. A small guard test fails if the
suite-wide mocks are removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/test-utils/appModuleMocks.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, test, vi } from 'vitest';
+import { goto } from '$app/navigation';
+import { page } from '$app/state';
+
+// Guards the suite-wide $app mocks in vitest.setup.ts (#3943). If they are
+// removed, tests that never mock $app themselves go back to loading
+// SvelteKit's client runtime, which fails at module load on some CI runs.
+describe('suite-wide SvelteKit app-module mocks', () => {
+  test('$app/navigation is the mock, not the client runtime', () => {
+    expect(vi.isMockFunction(goto)).toBe(true);
+  });
+
+  test('$app/state gives a page at the root URL', () => {
+    expect(page.url.pathname).toBe('/');
+  });
+});