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/lib/warn-once.js

@@ -0,0 +1,13 @@
+/* eslint-disable no-console */
+'use strict'
+
+let printed = {}
+
+module.exports = function warnOnce(message) {
+  if (printed[message]) return
+  printed[message] = true
+
+  if (typeof console !== 'undefined' && console.warn) {
+    console.warn(message)
+  }
+}