style(preview): double panel width, fix Chrome/Edge white scrollbar
- Panel width: 270px → 540px (2x) for better parameter display
- Scrollbar fix for Chrome/Edge:
- Replaced backdrop-filter (not supported on scrollbar pseudo-
elements) with solid background-color
- Added !important to override Windows Fluent overlay scrollbar
- Used standards-track scrollbar-color on :root (Chrome 121+)
- Neutral gray rgba(128,128,128,.30) — visible on any background
- Hover: rgba(128,128,128,.45)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>