UI redesign: Ask, Explore, SQL, System tabs

- Ask: natural language → AI generates SQL → DataFusion executes → results
  Shows the AI-over-data-lake story: schema introspection → LLM → query
- Explore: click dataset → schema + preview + AI-generated summary
- SQL: raw DataFusion SQL editor with Ctrl+Enter
- System: health grid testing all 5 services + embeddings + generation
- Example prompts for quick demo
- Dark theme with accent styling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

data/_catalog/manifests/fc046131-8f44-428a-af90-ebf6347d4cc6.json

@@ -0,0 +1,15 @@
+{
+  "id": "fc046131-8f44-428a-af90-ebf6347d4cc6",
+  "name": "u",
+  "schema_fingerprint": "from-ui",
+  "objects": [
+    {
+      "bucket": "data",
+      "key": "datasets/employees.parquet",
+      "size_bytes": 0,
+      "created_at": "2026-03-27T12:16:51.462058823Z"
+    }
+  ],
+  "created_at": "2026-03-27T12:16:51.462071311Z",
+  "updated_at": "2026-03-27T12:16:51.462071311Z"
+}