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-array/src/pairs.js

@@ -0,0 +1,15 @@
+export default function pairs(values, pairof = pair) {
+  const pairs = [];
+  let previous;
+  let first = false;
+  for (const value of values) {
+    if (first) pairs.push(pairof(previous, value));
+    previous = value;
+    first = true;
+  }
+  return pairs;
+}
+
+export function pair(a, b) {
+  return [a, b];
+}