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/dom-helpers/esm/scrollbarSize.js

@@ -0,0 +1,19 @@
+import canUseDOM from './canUseDOM';
+var size;
+export default function scrollbarSize(recalc) {
+  if (!size && size !== 0 || recalc) {
+    if (canUseDOM) {
+      var scrollDiv = document.createElement('div');
+      scrollDiv.style.position = 'absolute';
+      scrollDiv.style.top = '-9999px';
+      scrollDiv.style.width = '50px';
+      scrollDiv.style.height = '50px';
+      scrollDiv.style.overflow = 'scroll';
+      document.body.appendChild(scrollDiv);
+      size = scrollDiv.offsetWidth - scrollDiv.clientWidth;
+      document.body.removeChild(scrollDiv);
+    }
+  }
+
+  return size;
+}