feat: replace logo with quill feather design (orange theme)

- Dark warm amber background (radial gradient #1a0800 → #040100)
- Asymmetric feather vane: wide trailing edge (cubic bezier),
  narrow leading edge (quadratic bezier)
- Cream-to-orange-to-deep-amber gradient along feather axis
- Golden glowing shaft with nib highlight
- 10 subtle barb lines (5 trailing, 5 leading) for feather texture
- Orange glow orb + warm accent near nib
- Thin outer ring in brand orange
- Matches #f97316 VitePress theme throughout

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zhonghui