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/utils/create-custom-error.js

@@ -0,0 +1,14 @@
+export function createCustomError(name, message) {
+    // use Object.create(), because some VMs prevent setting line/column otherwise
+    // (iOS Safari 10 even throws an exception)
+    const error = Object.create(SyntaxError.prototype);
+    const errorStack = new Error();
+
+    return Object.assign(error, {
+        name,
+        message,
+        get stack() {
+            return (errorStack.stack || '').replace(/^(.+\n){1,3}/, `${name}: ${message}\n`);
+        }
+    });
+};