Add a GitHub Pages landing page for g (#290)

## Summary
- add a Vite-based landing page for `g` under `web/`
- style the page around a terminal-inspired GitHub Pages experience with
product messaging and screenshots
- add GitHub Pages workflow support and update README guidance for local
preview/deployment
- update release publishing workflow alongside the new site setup

## Testing
- `npm run build` (in `web/`)

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