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/AppDetailPage.tsx

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+export function AppDetailPage() {
+  return <div>App Detail</div>;
+}

ui/src/pages/DashboardPage.tsx

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+export function DashboardPage() {
+  return <div>Dashboard</div>;
+}

ui/src/pages/EnvironmentDetailPage.tsx

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+export function EnvironmentDetailPage() {
+  return <div>Environment Detail</div>;
+}

ui/src/pages/EnvironmentsPage.tsx

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+export function EnvironmentsPage() {
+  return <div>Environments</div>;
+}

ui/src/pages/LicensePage.tsx

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+export function LicensePage() {
+  return <div>License</div>;
+}