Sheet, ErrorBanner, Skeleton and EmptyState (#36)

Co-authored-by: dev_agent <dev_agent@cubecraftlabs.com>
Co-committed-by: dev_agent <dev_agent@cubecraftlabs.com>

web/src/ui/ErrorBanner.tsx

@@ -0,0 +1,21 @@
+import type { ReactNode } from 'react'
+import { Button } from './Button.tsx'
+import { Card } from './Card.tsx'
+import styles from './ErrorBanner.module.css'
+
+export type ErrorBannerProps = {
+  // What went wrong, in the caller's words.
+  children: ReactNode
+  onRetry: () => void
+  retryLabel?: string
+}
+
+// Muted neutral-100 card with grape-700 text and a Retry button.
+export function ErrorBanner({ children, onRetry, retryLabel = 'Retry' }: ErrorBannerProps) {
+  return (
+    <Card tone="muted" role="alert" className={styles.banner}>
+      <p className={styles.message}>{children}</p>
+      <Button onClick={onRetry}>{retryLabel}</Button>
+    </Card>
+  )
+}