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/App.module.css

@@ -0,0 +1,57 @@
+.shell {
+  display: flex;
+  height: 100vh;
+  background: var(--color-bg);
+  color: var(--color-text);
+  font-family: var(--font-family);
+  overflow: hidden;
+}
+
+.brand {
+  font-weight: 700;
+  font-size: 1rem;
+  color: var(--color-primary);
+  letter-spacing: 0.02em;
+}
+
+.nav {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  padding: var(--space-2) 0;
+}
+
+.navItem {
+  display: block;
+  width: 100%;
+  padding: var(--space-2) var(--space-3);
+  background: none;
+  border: none;
+  border-radius: var(--radius-md);
+  color: var(--color-text);
+  font-size: var(--font-size-sm);
+  font-family: var(--font-family);
+  text-align: left;
+  cursor: pointer;
+  transition: background 120ms ease, color 120ms ease;
+}
+
+.navItem:hover {
+  background: var(--color-bg-subtle);
+}
+
+.navItemActive {
+  background: var(--color-secondary);
+  color: var(--color-secondary-fg);
+  font-weight: 600;
+}
+
+.navItemActive:hover {
+  background: var(--color-secondary-hover);
+}
+
+.main {
+  flex: 1;
+  overflow-y: auto;
+  padding: var(--space-6);
+}