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/autoprefixer/lib/hacks/placeholder-shown.js

@@ -0,0 +1,19 @@
+let Selector = require('../selector')
+
+class PlaceholderShown extends Selector {
+  /**
+   * Return different selectors depend on prefix
+   */
+  prefixed(prefix) {
+    if (prefix === '-moz-') {
+      return ':-moz-placeholder'
+    } else if (prefix === '-ms-') {
+      return ':-ms-input-placeholder'
+    }
+    return `:${prefix}placeholder-shown`
+  }
+}
+
+PlaceholderShown.names = [':placeholder-shown']
+
+module.exports = PlaceholderShown