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/components/Sidebar.tsx

@@ -0,0 +1,106 @@
+import { Link, useLocation } from "@tanstack/react-router";
+import {
+  CreditCard,
+  Home,
+  List,
+  BarChart3,
+  Bell,
+  TrendingUp,
+  X,
+} from "lucide-react";
+import { useQuery } from "@tanstack/react-query";
+import { apiClient } from "../lib/api";
+import { formatCurrency } from "../lib/utils";
+import { cn } from "../lib/utils";
+import type { Account } from "../types/api";
+
+const navigation = [
+  { name: "Overview", icon: Home, to: "/" },
+  { name: "Transactions", icon: List, to: "/transactions" },
+  { name: "Analytics", icon: BarChart3, to: "/analytics" },
+  { name: "Notifications", icon: Bell, to: "/notifications" },
+];
+
+interface SidebarProps {
+  sidebarOpen: boolean;
+  setSidebarOpen: (open: boolean) => void;
+}
+
+export default function Sidebar({ sidebarOpen, setSidebarOpen }: SidebarProps) {
+  const location = useLocation();
+
+  const { data: accounts } = useQuery<Account[]>({
+    queryKey: ["accounts"],
+    queryFn: apiClient.getAccounts,
+  });
+
+  const totalBalance =
+    accounts?.reduce((sum, account) => {
+      const primaryBalance = account.balances?.[0]?.amount || 0;
+      return sum + primaryBalance;
+    }, 0) || 0;
+
+  return (
+    <div
+      className={cn(
+        "fixed inset-y-0 left-0 z-50 w-64 bg-white shadow-lg transform transition-transform duration-300 ease-in-out lg:translate-x-0 lg:static lg:inset-0",
+        sidebarOpen ? "translate-x-0" : "-translate-x-full",
+      )}
+    >
+      <div className="flex items-center justify-between h-16 px-6 border-b border-gray-200">
+        <Link
+          to="/"
+          onClick={() => setSidebarOpen(false)}
+          className="flex items-center space-x-2 hover:opacity-80 transition-opacity"
+        >
+          <CreditCard className="h-8 w-8 text-blue-600" />
+          <h1 className="text-xl font-bold text-gray-900">Leggen</h1>
+        </Link>
+        <button
+          onClick={() => setSidebarOpen(false)}
+          className="lg:hidden p-1 rounded-md text-gray-400 hover:text-gray-500"
+        >
+          <X className="h-6 w-6" />
+        </button>
+      </div>
+
+      <nav className="px-6 py-4">
+        <div className="space-y-1">
+          {navigation.map((item) => (
+            <Link
+              key={item.to}
+              to={item.to}
+              onClick={() => setSidebarOpen(false)}
+              className={`flex items-center w-full px-3 py-2 text-sm font-medium rounded-md transition-colors ${
+                location.pathname === item.to
+                  ? "bg-blue-100 text-blue-700"
+                  : "text-gray-700 hover:text-gray-900 hover:bg-gray-100"
+              }`}
+            >
+              <item.icon className="mr-3 h-5 w-5" />
+              {item.name}
+            </Link>
+          ))}
+        </div>
+      </nav>
+
+      {/* Account Summary in Sidebar */}
+      <div className="px-6 py-4 border-t border-gray-200 mt-auto">
+        <div className="bg-gray-50 rounded-lg p-4">
+          <div className="flex items-center justify-between">
+            <span className="text-sm font-medium text-gray-600">
+              Total Balance
+            </span>
+            <TrendingUp className="h-4 w-4 text-green-500" />
+          </div>
+          <p className="text-2xl font-bold text-gray-900 mt-1">
+            {formatCurrency(totalBalance)}
+          </p>
+          <p className="text-sm text-gray-500 mt-1">
+            {accounts?.length || 0} accounts
+          </p>
+        </div>
+      </div>
+    </div>
+  );
+}