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/d3-shape/src/symbol/diamond2.js

@@ -0,0 +1,12 @@
+import {sqrt} from "../math.js";
+
+export default {
+  draw(context, size) {
+    const r = sqrt(size) * 0.62625;
+    context.moveTo(0, -r);
+    context.lineTo(r, 0);
+    context.lineTo(0, r);
+    context.lineTo(-r, 0);
+    context.closePath();
+  }
+};