feat(client): add UI kit, data layer, app shell, and Table component

- design token system (Atlantis, Kimberly, Powder Ash palette)
- Button, Badge, Input, Select, Card, SidePanel, Breadcrumbs components
- generic typed Table<T> component with loading/empty states
- API data layer: typed fetch client for environments, keys, sessions, scenarios
- Vite dev proxy targeting server on port 13000
- App shell with SidePanel nav and four entity pages (Environments, Keys, Sessions, Scenarios)
- Storybook config with dark/light theme toggle and a11y addon

client/src/declarations.d.ts

@@ -0,0 +1,8 @@
+declare module '*.css' {
+  const styles: Record<string, string>;
+  export default styles;
+}
+
+interface ImportMetaEnv {
+  readonly VITE_API_URL?: string;
+}