fix(design-sync): address CodeRabbit review on preview files

- Add `import type * as React from "react"` to the 26 previews that reference
  `React.CSSProperties` under the automatic JSX runtime (React isn't a global
  type namespace there, so the annotation was unresolved standalone).
- Accordion preview: use `multiple` instead of `openMultiple` — @base-ui/react
  1.6 renamed the prop, so the multi-open cell wasn't actually multi-open.

Skipped CodeRabbit's BrandIcon dark-mode note: the preview renders in the
default light card (the dark <img> is hidden there); per-theme sources would
need component support it doesn't expose.

Claude-Session: https://claude.ai/code/session_01R8Aq8F1nTuvJwfut7g3DVE

.design-sync/previews/Slider.tsx

@@ -1,3 +1,4 @@
+import type * as React from "react";
 import { Label } from "@reactive-resume/ui/components/label";
 import { Slider } from "@reactive-resume/ui/components/slider";