feat: ✨ smooth curves + interactive hover for data_visualization charts

- Line and area charts now use monotone-cubic (Fritsch–Carlson) interpolation
  for smooth curves that never overshoot the data (safe for negatives).
- Hover behavior matching Slack:
  - line/bar show a tooltip with the category and every series value, plus a
    dashed vertical guide at the nearest category; line/area also mark each
    series' value at that point.
  - pie shows a tooltip for the hovered segment, and the slice expands outward
    from the pie center.
- All interactivity is client-side and SSR-safe (default render is unchanged).
- Add demo screenshots under .github/assets (excluded from the npm package).

https://claude.ai/code/session_018T7Lik8eFCV8DMtTckZC6a

Author: claude