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/ui/index.ts

@@ -0,0 +1,23 @@
+export { Button } from './Button/Button';
+export type { ButtonProps } from './Button/Button';
+
+export { Badge } from './Badge/Badge';
+export type { BadgeProps, BadgeVariant } from './Badge/Badge';
+
+export { Input } from './Input/Input';
+export type { InputProps } from './Input/Input';
+
+export { Select } from './Select/Select';
+export type { SelectProps, SelectOption } from './Select/Select';
+
+export { Card } from './Card/Card';
+export type { CardProps } from './Card/Card';
+
+export { SidePanel } from './SidePanel/SidePanel';
+export type { SidePanelProps } from './SidePanel/SidePanel';
+
+export { Breadcrumbs } from './Breadcrumbs/Breadcrumbs';
+export type { BreadcrumbsProps, BreadcrumbItem } from './Breadcrumbs/Breadcrumbs';
+
+export { Table } from './Table/Table';
+export type { TableProps, TableColumn } from './Table/Table';