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/pages/EnvironmentsPage.tsx

@@ -0,0 +1,3 @@
+export function EnvironmentsPage() {
+  return <div>Environments</div>;
+}