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/stop-iteration-iterator/index.d.ts

@@ -0,0 +1,7 @@
+declare global {
+    const StopIteration: (object & { readonly __brand__?: unique symbol }) | undefined;
+}
+
+declare function getStopIterationIterator<T>(origIterator: { next(): IteratorResult<T> }): Iterator<T, unknown, T>;
+
+export = getStopIterationIterator;