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/css-tree/lib/syntax/node/NestingSelector.js

@@ -0,0 +1,22 @@
+import { Delim } from '../../tokenizer/index.js';
+
+const AMPERSAND = 0x0026;       // U+0026 AMPERSAND (&)
+
+export const name = 'NestingSelector';
+export const structure = {
+};
+
+export function parse() {
+    const start = this.tokenStart;
+
+    this.eatDelim(AMPERSAND);
+
+    return {
+        type: 'NestingSelector',
+        loc: this.getLocation(start, this.tokenStart)
+    };
+}
+
+export function generate() {
+    this.token(Delim, '&');
+}