redesign serve viewer: Notion-style, nutshell branding

- Embed nutshell SVG icon in nav brand (icon + bicolor nut|shell)
- Notion-inspired design: Inter + JetBrains Mono fonts, #165DFF blue,
  cream/off-white backgrounds, hairline borders, frosted glass nav
- Hero section with title, summary, tag badges
- Professional card layout: 1px-gap grid, collapsible manifest
- Zero emoji — all icons are inline SVGs
- Checklist with checkbox UI, constraints with dot markers
- File list with file-icon per entry, click-to-preview in directory mode

Author: inksong