Add Cypress E2E tests with fixture-based API mocking for UI regression protection

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

cypress/fixtures/api/token.json

@@ -0,0 +1,6 @@
+{
+  "access_token": "fake-access-token-for-testing",
+  "token_type": "bearer",
+  "expires_in": 3600,
+  "refresh_token": "fake-refresh-token-for-testing"
+}