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/assertion-error/index.d.ts

@@ -0,0 +1,11 @@
+type AssertionError<T = {}> = Error & T & {
+    showDiff: boolean;
+};
+
+interface AssertionErrorConstructor {
+    new<T = {}>(message: string, props?: T, ssf?: Function): AssertionError<T>;
+}
+
+declare const AssertionError: AssertionErrorConstructor;
+
+export = AssertionError;