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/users.json

@@ -0,0 +1,26 @@
+[
+  {
+    "username": "testadmin",
+    "name": "Test Admin",
+    "is_admin": true,
+    "is_active": true,
+    "is_sso": false,
+    "has_data": true
+  },
+  {
+    "username": "user1",
+    "name": "Alice Wang",
+    "is_admin": false,
+    "is_active": true,
+    "is_sso": false,
+    "has_data": true
+  },
+  {
+    "username": "user2",
+    "name": "Bob Chen",
+    "is_admin": false,
+    "is_active": false,
+    "is_sso": true,
+    "has_data": false
+  }
+]