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/ui/ErrorBanner.module.css

@@ -0,0 +1,16 @@
+.banner {
+  display: flex;
+  align-items: center;
+  gap: var(--space-3);
+  background: var(--color-neutral-100);
+}
+
+.message {
+  flex: 1;
+  margin: 0;
+  color: var(--color-accent-700);
+  font-family: var(--font-body);
+  font-size: 15px;
+  font-weight: 600;
+  line-height: 1.35;
+}