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/lucide-react/dist/esm/icons/mails.js

@@ -0,0 +1,17 @@
+/**
+ * @license lucide-react v0.303.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */
+
+import createLucideIcon from '../createLucideIcon.js';
+
+const Mails = createLucideIcon("Mails", [
+  ["rect", { width: "16", height: "13", x: "6", y: "4", rx: "2", key: "1drq3f" }],
+  ["path", { d: "m22 7-7.1 3.78c-.57.3-1.23.3-1.8 0L6 7", key: "xn252p" }],
+  ["path", { d: "M2 8v11c0 1.1.9 2 2 2h14", key: "n13cji" }]
+]);
+
+export { Mails as default };
+//# sourceMappingURL=mails.js.map