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/data-url.js

@@ -0,0 +1,17 @@
+"use strict"
+
+const dataURLRegexp = /^data:text\/css;base64,/i
+
+function isValid(url) {
+  return dataURLRegexp.test(url)
+}
+
+function contents(url) {
+  // "data:text/css;base64,".length === 21
+  return Buffer.from(url.slice(21), "base64").toString()
+}
+
+module.exports = {
+  isValid,
+  contents,
+}