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/form-data/lib/populate.js

@@ -0,0 +1,10 @@
+'use strict';
+
+// populates missing values
+module.exports = function (dst, src) {
+  Object.keys(src).forEach(function (prop) {
+    dst[prop] = dst[prop] || src[prop]; // eslint-disable-line no-param-reassign
+  });
+
+  return dst;
+};