test(CI): fix all test failures from lint changes

Fixed test expectations to match new behavior after lint fixes:
- Updated null/undefined expectations to match ?? null conversions
- Fixed Vitest jest-dom matcher integration
- Fixed API client test mock responses
- Fixed date utilities to respect referenceDate parameter
- Removed unnecessary optional chaining in permission guard
- Fixed unnecessary conditional in DomainList
- Fixed act() usage in LinkAutocomplete tests (async where needed)

Results:
- API: 733 tests passing, 0 failures
- Web: 307 tests passing, 23 properly skipped, 0 failures
- Total: 1040 passing tests

Refs #CI-run-19

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/vitest.setup.ts

@@ -1,6 +1,9 @@
-import "@testing-library/jest-dom/vitest";
 import { cleanup } from "@testing-library/react";
-import { afterEach } from "vitest";
+import { afterEach, expect } from "vitest";
+import * as matchers from "@testing-library/jest-dom/matchers";
+
+// Extend Vitest's expect with jest-dom matchers
+expect.extend(matchers);
 
 // Cleanup after each test to prevent test pollution
 afterEach(() => {