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/object.assign/shim.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var define = require('define-properties');
+var getPolyfill = require('./polyfill');
+
+module.exports = function shimAssign() {
+	var polyfill = getPolyfill();
+	define(
+		Object,
+		{ assign: polyfill },
+		{ assign: function () { return Object.assign !== polyfill; } }
+	);
+	return polyfill;
+};