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/clsx/clsx.d.ts

@@ -0,0 +1,10 @@
+declare namespace clsx {
+	type ClassValue = ClassArray | ClassDictionary | string | number | bigint | null | boolean | undefined;
+	type ClassDictionary = Record<string, any>;
+	type ClassArray = ClassValue[];
+	function clsx(...inputs: ClassValue[]): string;
+}
+
+declare function clsx(...inputs: clsx.ClassValue[]): string;
+
+export = clsx;