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-format/src/formatDecimal.js

@@ -0,0 +1,20 @@
+export default function(x) {
+  return Math.abs(x = Math.round(x)) >= 1e21
+      ? x.toLocaleString("en").replace(/,/g, "")
+      : x.toString(10);
+}
+
+// Computes the decimal coefficient and exponent of the specified number x with
+// significant digits p, where x is positive and p is in [1, 21] or undefined.
+// For example, formatDecimalParts(1.23) returns ["123", 0].
+export function formatDecimalParts(x, p) {
+  if (!isFinite(x) || x === 0) return null; // NaN, ±Infinity, ±0
+  var i = (x = p ? x.toExponential(p - 1) : x.toExponential()).indexOf("e"), coefficient = x.slice(0, i);
+
+  // The string returned by toExponential either has the form \d\.\d+e[-+]\d+
+  // (e.g., 1.2e+3) or the form \de[-+]\d+ (e.g., 1e+3).
+  return [
+    coefficient.length > 1 ? coefficient[0] + coefficient.slice(2) : coefficient,
+    +x.slice(i + 1)
+  ];
+}