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/traces.json

@@ -0,0 +1,16 @@
+{
+  "traces": [
+    {
+      "id": "trace-001",
+      "case_id": "CASE-001",
+      "count": 50,
+      "activities": ["Task A", "Task B", "End"]
+    },
+    {
+      "id": "trace-002",
+      "case_id": "CASE-002",
+      "count": 30,
+      "activities": ["Task A", "End"]
+    }
+  ]
+}