Add Sheet, ErrorBanner, Skeleton and EmptyState primitives

Sheet is a portalled modal bottom sheet: role="dialog" with aria-modal
and a required accessible name, focus moved in, trapped and restored on
close, Escape and scrim click call onClose, and the rest of <body> is
inert while open. It rises from translateY(100%) over 320 ms
var(--ease-out-soft) over a scrim fading to 50% neutral-900; under
prefers-reduced-motion it opens in place.

ErrorBanner is a muted neutral-100 Card with grape-700 text and a Retry
Button; Skeleton is a --color-surface pill with a 1.2 s opacity pulse
switched off under reduced motion; EmptyState has a Bagel title, body
and an optional primary Button action.

jsdom never matches prefers-reduced-motion, resolves var() colours to
transparent and does not expand the animation shorthand, so the new
src/test/cssRules.ts reads the CSS Modules' declared rules, the
reduced-motion media block and the hashed @keyframes directly.

Maestro task: 2f9b64dd-0c26-47b9-b2b8-557bdf506d68

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/test/cssRules.ts

@@ -0,0 +1,50 @@
+// Reads the CSS Modules' rules straight from the stylesheets Vitest injects
+// (css: true). jsdom's getComputedStyle cannot answer two things these
+// primitives need checked: it resolves var() colours to transparent, and it
+// evaluates only "all" and "screen" media queries, so styles inside
+// @media (prefers-reduced-motion: reduce) never apply.
+
+function cssRules(): CSSRule[] {
+  return Array.from(document.styleSheets).flatMap((sheet) => Array.from(sheet.cssRules))
+}
+
+function collect(element: Element, rules: CSSRule[], style: CSSStyleDeclaration) {
+  for (const rule of rules) {
+    if (!(rule instanceof CSSStyleRule) || !element.matches(rule.selectorText)) continue
+    for (const property of Array.from(rule.style)) {
+      style.setProperty(property, rule.style.getPropertyValue(property))
+    }
+  }
+}
+
+// The declarations the stylesheets give the element, as written. Read the
+// shorthands the CSS uses (background, animation): jsdom does not expand
+// animation, nor any shorthand holding a var(). Later rules win; specificity
+// is not considered.
+export function declaredStyle(element: Element): CSSStyleDeclaration {
+  const style = document.createElement('div').style
+  collect(element, cssRules(), style)
+  return style
+}
+
+// Only the declarations @media (prefers-reduced-motion: reduce) gives the element.
+export function reducedMotionStyle(element: Element): CSSStyleDeclaration {
+  const style = document.createElement('div').style
+  for (const rule of cssRules()) {
+    if (rule instanceof CSSMediaRule && rule.conditionText.includes('prefers-reduced-motion: reduce')) {
+      collect(element, Array.from(rule.cssRules), style)
+    }
+  }
+  return style
+}
+
+// The @keyframes the element's animation runs. CSS Modules hash keyframe
+// names, so this follows the element's own animation-name.
+export function animationKeyframes(element: Element): CSSKeyframesRule {
+  const [name] = declaredStyle(element).animation.split(' ')
+  const keyframes = cssRules().find(
+    (rule): rule is CSSKeyframesRule => rule instanceof CSSKeyframesRule && rule.name === name,
+  )
+  if (!keyframes) throw new Error(`no @keyframes named ${JSON.stringify(name)}`)
+  return keyframes
+}