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/load-content.js

@@ -0,0 +1,12 @@
+"use strict"
+
+const readCache = require("read-cache")
+const dataURL = require("./data-url")
+
+module.exports = filename => {
+  if (dataURL.isValid(filename)) {
+    return dataURL.contents(filename)
+  }
+
+  return readCache(filename, "utf-8")
+}