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/my-account.json

@@ -0,0 +1,9 @@
+{
+  "username": "testadmin",
+  "name": "Test Admin",
+  "is_sso": false,
+  "created_at": "2025-01-15T10:00:00Z",
+  "roles": [
+    { "code": "admin", "name": "Administrator" }
+  ]
+}