Rebuild docs component preview and demos #64

The docs preview and demo code don't match the quality bar of other shadcn registries.

• Use Fumadocs' Preview / Code tabs and its standard code block (capped height, scroll, copy).
• Preview toolbar: shadcn accent theme picker and dark mode (shared across previews, remembered), reset, Open in v0 / Lovable / Bolt.
• Short demos: one component with its props, sample data in src/lib/elements-config.ts.
• Remove the custom preview shell and code panel.