docs: rewrite README for clarity and onboarding

Complete rewrite focused on getting anyone from zero to running app
in under two minutes. Key improvements:

- Clear two-mode overview table (Native Widgets vs WebView) up top
- Single-command install with zero-dep note for Linux
- Quick Start that fits on one screen
- Widget reference table with links to per-widget docs
- Styled/layout/dialog/menu code patterns in one place
- WebView section: bridge API, serveFromDisk, Vite, CORS proxy
- Platform support table with exact dep commands
- Flat doc link section at the bottom

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: developersharif