[Phase 1] Implement ChartDemo component and demo data

[afucher]

Create the reusable ChartDemo.astro component that renders interactive AstroChart demos in pages, plus a shared demoData.ts module.

Deliverable

A working chart demo component that can be dropped into any Markdown page and renders an interactive SVG chart with optional code snippet display.

Acceptance Criteria

• src/components/ChartDemo.astro implemented with props:
  • id: string — unique container ID
  • height?: number — default 500
  • mode?: 'radix' | 'transit' | 'animate' — chart type
  • showCode?: boolean — default true, shows code block
• Component renders a <div id={id}> container
• Inline <script is:inline> loads /public/astrochart.js and initializes chart
• For animate mode: renders a button that calls transit.animate() on click
• Code snippet display: collapsible <details> with syntax-highlighted code (via Shiki or Prism)
• <noscript> fallback showing placeholder message
• src/data/demoData.ts created with:
  • defaultRadixData: AstroData — 15 planets + 12 cusps
  • defaultTransitData: AstroData — transit ring planets + cusps
• Demo component tested in a test page (src/content/docs/test-demo.md)
• Chart renders visually correctly in dev server (no console errors)
• Component is documented with JSDoc comment explaining all props

Related Plan Section

docs-plan.md §6 (Visual & Interactive Examples)