feat: site polish - single column layout, install sections, platform icons

- Change bento grid to single column stacked layout
- Add mid-page install section after CLI logos (duplicated at bottom)
- Add platform icons (Windows, Mac, Tux) to install tabs
- Add Mac/Linux platform links in hero that scroll to install + select tab
- Auto-detect platform and pre-select correct install tab
- Update "existing logins" to "existing plans and API keys"
- Update locally-backed tasks text (already implemented)
- Update footer copyright to Neural Pixel with auto-updating year
- Add GitHub logo to Repository nav button and View on GitHub button
- Tighten section spacing throughout
- Improve footer link contrast and WEN ELON styling
- Change "Run the commands below" to "Run from Source ↓"

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

Author: web3dev1337