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/discover.json

@@ -0,0 +1,30 @@
+{
+  "stats": {
+    "cases": { "count": 150, "total": 200, "ratio": 0.75 },
+    "traces": { "count": 45, "total": 60, "ratio": 0.75 },
+    "task_instances": { "count": 1200, "total": 1500, "ratio": 0.8 },
+    "tasks": { "count": 12, "total": 15, "ratio": 0.8 },
+    "started_at": "2025-01-01T00:00:00Z",
+    "completed_at": "2025-06-01T00:00:00Z",
+    "case_duration": {
+      "min": 3600,
+      "max": 864000,
+      "average": 172800,
+      "median": 86400
+    }
+  },
+  "graph": {
+    "nodes": [
+      { "id": "start", "label": "Start", "type": "start" },
+      { "id": "task_a", "label": "Task A", "type": "task" },
+      { "id": "task_b", "label": "Task B", "type": "task" },
+      { "id": "end", "label": "End", "type": "end" }
+    ],
+    "edges": [
+      { "source": "start", "target": "task_a", "count": 150 },
+      { "source": "task_a", "target": "task_b", "count": 120 },
+      { "source": "task_b", "target": "end", "count": 120 },
+      { "source": "task_a", "target": "end", "count": 30 }
+    ]
+  }
+}