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/pages/Page.module.css

@@ -0,0 +1,15 @@
+.heading {
+  margin: 0 0 var(--space-4);
+  font-size: var(--font-size-lg);
+  font-weight: 700;
+  color: var(--color-text);
+}
+
+.error {
+  margin-bottom: var(--space-4);
+  padding: var(--space-3) var(--space-4);
+  background: var(--color-error-bg);
+  color: var(--color-error-fg);
+  border-radius: var(--radius-md);
+  font-size: var(--font-size-sm);
+}