diff --git a/ui/src/components/EnvironmentTree.tsx b/ui/src/components/EnvironmentTree.tsx
new file mode 100644
index 0000000..a0db25b
--- /dev/null
+++ b/ui/src/components/EnvironmentTree.tsx
@@ -0,0 +1,114 @@
+import { useState, useCallback } from 'react';
+import { useNavigate, useLocation } from 'react-router';
+import { SidebarTree, type SidebarTreeNode } from '@cameleer/design-system';
+import { useAuthStore } from '../auth/auth-store';
+import { useEnvironments, useApps } from '../api/hooks';
+import type { EnvironmentResponse } from '../types/api';
+
+/**
+ * Renders one environment entry as a SidebarTreeNode.
+ * This is a "render nothing, report data" component: it fetches apps for
+ * the given environment and invokes `onNode` with the assembled tree node
+ * whenever the data changes.
+ *
+ * Using a dedicated component per env is the idiomatic way to call a hook
+ * for each item in a dynamic list without violating Rules of Hooks.
+ */
+function EnvWithApps({
+  env,
+  onNode,
+}: {
+  env: EnvironmentResponse;
+  onNode: (node: SidebarTreeNode) => void;
+}) {
+  const { data: apps } = useApps(env.id);
+
+  const children: SidebarTreeNode[] = (apps ?? []).map((app) => ({
+    id: app.id,
+    label: app.displayName,
+    path: `/environments/${env.id}/apps/${app.id}`,
+  }));
+
+  const node: SidebarTreeNode = {
+    id: env.id,
+    label: env.displayName,
+    path: `/environments/${env.id}`,
+    children: children.length > 0 ? children : undefined,
+  };
+
+  // Calling onNode during render is intentional here: we want the parent to
+  // collect the latest node on every render. The parent guards against
+  // infinite loops by doing a shallow equality check before updating state.
+  onNode(node);
+
+  return null;
+}
+
+export function EnvironmentTree() {
+  const tenantId = useAuthStore((s) => s.tenantId);
+  const { data: environments } = useEnvironments(tenantId ?? '');
+  const navigate = useNavigate();
+  const location = useLocation();
+
+  const [starred, setStarred] = useState<Set<string>>(new Set());
+  const [envNodes, setEnvNodes] = useState<Map<string, SidebarTreeNode>>(new Map());
+
+  const handleToggleStar = useCallback((id: string) => {
+    setStarred((prev) => {
+      const next = new Set(prev);
+      if (next.has(id)) {
+        next.delete(id);
+      } else {
+        next.add(id);
+      }
+      return next;
+    });
+  }, []);
+
+  const handleNode = useCallback((node: SidebarTreeNode) => {
+    setEnvNodes((prev) => {
+      const existing = prev.get(node.id);
+      // Avoid infinite re-renders: only update when something meaningful changed.
+      if (
+        existing &&
+        existing.label === node.label &&
+        existing.path === node.path &&
+        existing.children?.length === node.children?.length
+      ) {
+        return prev;
+      }
+      return new Map(prev).set(node.id, node);
+    });
+  }, []);
+
+  const envs = environments ?? [];
+
+  // Build the final node list, falling back to env-only nodes until apps load.
+  const nodes: SidebarTreeNode[] = envs.map(
+    (env) =>
+      envNodes.get(env.id) ?? {
+        id: env.id,
+        label: env.displayName,
+        path: `/environments/${env.id}`,
+      },
+  );
+
+  return (
+    <>
+      {/* Invisible data-fetchers: one per environment */}
+      {envs.map((env) => (
+        <EnvWithApps key={env.id} env={env} onNode={handleNode} />
+      ))}
+
+      <SidebarTree
+        nodes={nodes}
+        selectedPath={location.pathname}
+        isStarred={(id) => starred.has(id)}
+        onToggleStar={handleToggleStar}
+        onNavigate={(path) => navigate(path)}
+        persistKey="env-tree"
+        autoRevealPath={location.pathname}
+      />
+    </>
+  );
+}
diff --git a/ui/src/components/Layout.tsx b/ui/src/components/Layout.tsx
new file mode 100644
index 0000000..9df1430
--- /dev/null
+++ b/ui/src/components/Layout.tsx
@@ -0,0 +1,166 @@
+import { useState } from 'react';
+import { Outlet, useNavigate } from 'react-router';
+import {
+  AppShell,
+  Sidebar,
+  TopBar,
+} from '@cameleer/design-system';
+import { useAuthStore } from '../auth/auth-store';
+import { EnvironmentTree } from './EnvironmentTree';
+
+// Simple SVG logo mark for the sidebar header
+function CameleerLogo() {
+  return (
+    <svg
+      width="24"
+      height="24"
+      viewBox="0 0 24 24"
+      fill="none"
+      xmlns="http://www.w3.org/2000/svg"
+      aria-hidden="true"
+    >
+      <circle cx="12" cy="12" r="10" fill="currentColor" opacity="0.15" />
+      <path
+        d="M7 14c0-2.5 2-4.5 4.5-4.5S16 11.5 16 14"
+        stroke="currentColor"
+        strokeWidth="2"
+        strokeLinecap="round"
+      />
+      <circle cx="12" cy="8" r="2" fill="currentColor" />
+    </svg>
+  );
+}
+
+// Nav icon helpers
+function DashboardIcon() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+      <rect x="1" y="1" width="6" height="6" rx="1" fill="currentColor" />
+      <rect x="9" y="1" width="6" height="6" rx="1" fill="currentColor" />
+      <rect x="1" y="9" width="6" height="6" rx="1" fill="currentColor" />
+      <rect x="9" y="9" width="6" height="6" rx="1" fill="currentColor" />
+    </svg>
+  );
+}
+
+function EnvIcon() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+      <path
+        d="M2 4h12M2 8h12M2 12h12"
+        stroke="currentColor"
+        strokeWidth="1.5"
+        strokeLinecap="round"
+      />
+    </svg>
+  );
+}
+
+function LicenseIcon() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+      <rect x="2" y="2" width="12" height="12" rx="2" stroke="currentColor" strokeWidth="1.5" />
+      <path d="M5 8h6M5 5h6M5 11h4" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
+    </svg>
+  );
+}
+
+function ObsIcon() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+      <circle cx="8" cy="8" r="6" stroke="currentColor" strokeWidth="1.5" />
+      <path d="M4 8h8" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
+      <path d="M8 4v8" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
+    </svg>
+  );
+}
+
+function UserIcon() {
+  return (
+    <svg width="16" height="16" viewBox="0 0 16 16" fill="none" aria-hidden="true">
+      <circle cx="8" cy="5" r="3" stroke="currentColor" strokeWidth="1.5" />
+      <path
+        d="M2 13c0-3 2.7-5 6-5s6 2 6 5"
+        stroke="currentColor"
+        strokeWidth="1.5"
+        strokeLinecap="round"
+      />
+    </svg>
+  );
+}
+
+export function Layout() {
+  const navigate = useNavigate();
+  const username = useAuthStore((s) => s.username);
+  const logout = useAuthStore((s) => s.logout);
+
+  const [envSectionOpen, setEnvSectionOpen] = useState(true);
+  const [collapsed, setCollapsed] = useState(false);
+
+  const sidebar = (
+    <Sidebar collapsed={collapsed} onCollapseToggle={() => setCollapsed((c) => !c)}>
+      <Sidebar.Header
+        logo={<CameleerLogo />}
+        title="Cameleer SaaS"
+        onClick={() => navigate('/')}
+      />
+
+      {/* Dashboard */}
+      <Sidebar.Section
+        icon={<DashboardIcon />}
+        label="Dashboard"
+        open={false}
+        onToggle={() => navigate('/')}
+      >
+        {null}
+      </Sidebar.Section>
+
+      {/* Environments — expandable tree */}
+      <Sidebar.Section
+        icon={<EnvIcon />}
+        label="Environments"
+        open={envSectionOpen}
+        onToggle={() => setEnvSectionOpen((o) => !o)}
+      >
+        <EnvironmentTree />
+      </Sidebar.Section>
+
+      {/* License */}
+      <Sidebar.Section
+        icon={<LicenseIcon />}
+        label="License"
+        open={false}
+        onToggle={() => navigate('/license')}
+      >
+        {null}
+      </Sidebar.Section>
+
+      <Sidebar.Footer>
+        {/* Link to the observability SPA (external) */}
+        <Sidebar.FooterLink
+          icon={<ObsIcon />}
+          label="View Dashboard"
+          onClick={() => window.open('/dashboard', '_blank', 'noopener')}
+        />
+
+        {/* User info + logout */}
+        <Sidebar.FooterLink
+          icon={<UserIcon />}
+          label={username ?? 'Account'}
+          onClick={logout}
+        />
+      </Sidebar.Footer>
+    </Sidebar>
+  );
+
+  return (
+    <AppShell sidebar={sidebar}>
+      <TopBar
+        breadcrumb={[]}
+        user={username ? { name: username } : undefined}
+        onLogout={logout}
+      />
+      <Outlet />
+    </AppShell>
+  );
+}
diff --git a/ui/src/main.tsx b/ui/src/main.tsx
index ff6b27e..a255f95 100644
--- a/ui/src/main.tsx
+++ b/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>,
 );
diff --git a/ui/src/pages/AppDetailPage.tsx b/ui/src/pages/AppDetailPage.tsx
new file mode 100644
index 0000000..a5e44ad
--- /dev/null
+++ b/ui/src/pages/AppDetailPage.tsx
@@ -0,0 +1,3 @@
+export function AppDetailPage() {
+  return <div>App Detail</div>;
+}
diff --git a/ui/src/pages/DashboardPage.tsx b/ui/src/pages/DashboardPage.tsx
new file mode 100644
index 0000000..00ff2a0
--- /dev/null
+++ b/ui/src/pages/DashboardPage.tsx
@@ -0,0 +1,3 @@
+export function DashboardPage() {
+  return <div>Dashboard</div>;
+}
diff --git a/ui/src/pages/EnvironmentDetailPage.tsx b/ui/src/pages/EnvironmentDetailPage.tsx
new file mode 100644
index 0000000..ac61aff
--- /dev/null
+++ b/ui/src/pages/EnvironmentDetailPage.tsx
@@ -0,0 +1,3 @@
+export function EnvironmentDetailPage() {
+  return <div>Environment Detail</div>;
+}
diff --git a/ui/src/pages/EnvironmentsPage.tsx b/ui/src/pages/EnvironmentsPage.tsx
new file mode 100644
index 0000000..8e19427
--- /dev/null
+++ b/ui/src/pages/EnvironmentsPage.tsx
@@ -0,0 +1,3 @@
+export function EnvironmentsPage() {
+  return <div>Environments</div>;
+}
diff --git a/ui/src/pages/LicensePage.tsx b/ui/src/pages/LicensePage.tsx
new file mode 100644
index 0000000..47f25bd
--- /dev/null
+++ b/ui/src/pages/LicensePage.tsx
@@ -0,0 +1,3 @@
+export function LicensePage() {
+  return <div>License</div>;
+}
diff --git a/ui/src/router.tsx b/ui/src/router.tsx
new file mode 100644
index 0000000..e6b625a
--- /dev/null
+++ b/ui/src/router.tsx
@@ -0,0 +1,39 @@
+import { Routes, Route } from 'react-router';
+import { useEffect } from 'react';
+import { useAuthStore } from './auth/auth-store';
+import { LoginPage } from './auth/LoginPage';
+import { CallbackPage } from './auth/CallbackPage';
+import { ProtectedRoute } from './auth/ProtectedRoute';
+import { Layout } from './components/Layout';
+import { DashboardPage } from './pages/DashboardPage';
+import { EnvironmentsPage } from './pages/EnvironmentsPage';
+import { EnvironmentDetailPage } from './pages/EnvironmentDetailPage';
+import { AppDetailPage } from './pages/AppDetailPage';
+import { LicensePage } from './pages/LicensePage';
+
+export function AppRouter() {
+  const loadFromStorage = useAuthStore((s) => s.loadFromStorage);
+  useEffect(() => {
+    loadFromStorage();
+  }, [loadFromStorage]);
+
+  return (
+    <Routes>
+      <Route path="/login" element={<LoginPage />} />
+      <Route path="/callback" element={<CallbackPage />} />
+      <Route
+        element={
+          <ProtectedRoute>
+            <Layout />
+          </ProtectedRoute>
+        }
+      >
+        <Route index element={<DashboardPage />} />
+        <Route path="environments" element={<EnvironmentsPage />} />
+        <Route path="environments/:envId" element={<EnvironmentDetailPage />} />
+        <Route path="environments/:envId/apps/:appId" element={<AppDetailPage />} />
+        <Route path="license" element={<LicensePage />} />
+      </Route>
+    </Routes>
+  );
+}