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/performance.json

@@ -0,0 +1,18 @@
+{
+  "cycle_time": {
+    "labels": ["Task A", "Task B"],
+    "data": [7200, 3600]
+  },
+  "processing_time": {
+    "labels": ["Task A", "Task B"],
+    "data": [5400, 2700]
+  },
+  "waiting_time": {
+    "labels": ["Task A → Task B", "Task B → End"],
+    "data": [1800, 900]
+  },
+  "frequency": {
+    "labels": ["Task A", "Task B"],
+    "data": [150, 120]
+  }
+}