feat(frontend): add API client, layout, and routing

- Create Axios-based API client with sources, jobs, and dashboard endpoints
- Add responsive Layout component with navigation sidebar
- Set up React Router with Dashboard, Backups, and Settings routes
- Configure TanStack Query provider with default options
- Use lucide-react for navigation icons

frontend/node_modules/mdn-data/css/units.schema.json

@@ -0,0 +1,28 @@
+{
+  "type": "object",
+  "additionalProperties": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {
+      "groups": {
+        "type": "array",
+        "minitems": 1,
+        "uniqueItems": true,
+        "items": {
+          "$ref": "definitions.json#/groupList"
+        }
+      },
+      "status": {
+        "enum": [
+          "standard",
+          "nonstandard",
+          "experimental"
+        ]
+      }
+    },
+    "required": [
+      "groups",
+      "status"
+    ]
+  }
+}