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/Skeleton.tsx

@@ -0,0 +1,12 @@
+import type { ComponentPropsWithRef } from 'react'
+import styles from './Skeleton.module.css'
+
+export type SkeletonProps = Omit<ComponentPropsWithRef<'div'>, 'children'>
+
+// Loading placeholder: a pill in --color-surface with a 1.2 s opacity pulse.
+// Full width and 16 px tall by default; size it with className or style.
+// Hidden from assistive technology — announce loading on the container.
+export function Skeleton({ className, ...rest }: SkeletonProps) {
+  const classes = [styles.skeleton, className].filter(Boolean).join(' ')
+  return <div aria-hidden="true" className={classes} {...rest} />
+}