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/postcss-import/lib/join-layer.js

@@ -0,0 +1,9 @@
+"use strict"
+
+module.exports = function (parentLayer, childLayer) {
+  if (!parentLayer.length && childLayer.length) return childLayer
+  if (parentLayer.length && !childLayer.length) return parentLayer
+  if (!parentLayer.length && !childLayer.length) return []
+
+  return parentLayer.concat(childLayer)
+}