feat: add sidebar layout, environment tree, and router

Wires up AppShell + Sidebar compound component, a per-environment
SidebarTree that lazy-fetches apps, React Router nested routes, and
provider-wrapped main.tsx with ThemeProvider/ToastProvider/BreadcrumbProvider.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/main.tsx

@@ -1,8 +1,32 @@
 import React from 'react';
 import ReactDOM from 'react-dom/client';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { BrowserRouter } from 'react-router';
+import { ThemeProvider, ToastProvider, BreadcrumbProvider } from '@cameleer/design-system';
+import '@cameleer/design-system/style.css';
+import { AppRouter } from './router';
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      retry: 1,
+      staleTime: 10_000,
+    },
+  },
+});
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
-    <div>Cameleer SaaS</div>
-  </React.StrictMode>
+    <ThemeProvider>
+      <ToastProvider>
+        <BreadcrumbProvider>
+          <QueryClientProvider client={queryClient}>
+            <BrowserRouter>
+              <AppRouter />
+            </BrowserRouter>
+          </QueryClientProvider>
+        </BreadcrumbProvider>
+      </ToastProvider>
+    </ThemeProvider>
+  </React.StrictMode>,
 );