feat(crypto): add AES-GCM encryption utilities

- Add arrayToBase64/base64ToArray conversion functions
- Add deriveKey for PBKDF2 key derivation
- Add encrypt/decrypt using AES-GCM
- Add generateMasterKey for random key generation
- Update setup.ts to use real Web Crypto API instead of mock
- Add comprehensive unit tests for all crypto functions

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

desktop/tests/setup.ts

@@ -56,21 +56,11 @@ Object.defineProperty(global, 'localStorage', {
   configurable: true,
 });
 
-// Mock crypto.subtle for tests (already set via webcrypto above, but ensure subtle mock works)
-const subtleMock = {
-  encrypt: vi.fn(),
-  decrypt: vi.fn(),
-  generateKey: vi.fn(),
-  deriveKey: vi.fn(),
-  importKey: vi.fn(),
-  exportKey: vi.fn(),
-  digest: vi.fn(),
-  sign: vi.fn(),
-  verify: vi.fn(),
-};
-
-// Override subtle if needed for specific test scenarios
-Object.defineProperty(global.crypto, 'subtle', {
-  value: subtleMock,
-  configurable: true,
-});
+// Note: We intentionally do NOT mock crypto.subtle here.
+// Tests that need real crypto operations (like crypto-utils) will use the real Web Crypto API.
+// Tests that need to mock crypto operations should do so in their own test files.
+//
+// If you need to mock crypto.subtle for specific tests, use:
+//   vi.spyOn(crypto.subtle, 'encrypt').mockImplementation(...)
+// Or restore after mocking:
+//   afterAll(() => vi.restoreAllMocks())