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/Breadcrumbs/Breadcrumbs.module.css

@@ -0,0 +1,54 @@
+.nav {
+  font-family: var(--font-family);
+}
+
+.list {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: var(--space-1);
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.item {
+  display: flex;
+  align-items: center;
+  gap: var(--space-1);
+}
+
+.link {
+  font-size: var(--font-size-sm);
+  color: var(--color-text-muted);
+  text-decoration: none;
+  background: none;
+  border: none;
+  padding: 0;
+  cursor: pointer;
+  font-family: inherit;
+  border-radius: var(--radius-sm);
+  transition: color var(--transition);
+}
+
+.link:hover {
+  color: var(--color-text);
+  text-decoration: underline;
+}
+
+.link:focus-visible {
+  outline: 2px solid var(--color-primary);
+  outline-offset: 2px;
+}
+
+.current {
+  font-size: var(--font-size-sm);
+  font-weight: 600;
+  color: var(--color-text);
+}
+
+.separator {
+  display: flex;
+  align-items: center;
+  color: var(--color-border);
+}