feat(frontend): implement TanStack Router with mobile sidebar

- Install and configure TanStack Router for type-safe routing
- Create route structure with __root.tsx layout and individual route files
- Implement mobile-responsive sidebar with collapse functionality
- Add clickable logo in sidebar that navigates to overview page
- Extract Header and Sidebar components from Dashboard for reusability
- Configure Vite with TanStack Router plugin for development
- Update main.tsx to use RouterProvider instead of direct App rendering
- Maintain existing TanStack Query integration seamlessly
- Add proper TypeScript types for all route components
- Implement responsive design with mobile overlay and hamburger menu

This replaces the tab-based navigation with URL-based routing while
maintaining the same user experience and adding powerful features like:
- Bookmarkable URLs (/transactions, /analytics, /notifications)
- Browser back/forward button support
- Direct linking capabilities
- Mobile-responsive sidebar with smooth animations
- Type-safe navigation with auto-completion

frontend/src/main.tsx

@@ -1,10 +1,25 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
+import { createRouter, RouterProvider } from "@tanstack/react-router";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import "./index.css";
-import App from "./App.tsx";
+import { routeTree } from "./routeTree.gen";
+
+const router = createRouter({ routeTree });
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      refetchOnWindowFocus: false,
+      retry: 1,
+    },
+  },
+});
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
-    <App />
+    <QueryClientProvider client={queryClient}>
+      <RouterProvider router={router} />
+    </QueryClientProvider>
   </StrictMode>,
 );