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/tailwindcss/lib/util/isPlainObject.js

@@ -0,0 +1,17 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", {
+    value: true
+});
+Object.defineProperty(exports, "default", {
+    enumerable: true,
+    get: function() {
+        return isPlainObject;
+    }
+});
+function isPlainObject(value) {
+    if (Object.prototype.toString.call(value) !== "[object Object]") {
+        return false;
+    }
+    const prototype = Object.getPrototypeOf(value);
+    return prototype === null || Object.getPrototypeOf(prototype) === null;
+}