feat(docs): website hero redesign with FAA background and gallery page

[o4oren]

Summary

• Replaces the Cayman green header on all pages with a custom hero: FAA IFR chart as dimmed background + app screenshot composite as hero image
• Moves all screenshot grids from index.md into a new dedicated Gallery page (/gallery)
• Adds App Store and Google Play badges to the custom hero layout

Changes

• docs/_includes/head-custom.html — loads home-custom.css on every page (Cayman includes this automatically)
• docs/_layouts/home.html — custom Jekyll layout for homepage (hero structure)
• docs/assets/css/home-custom.css — hero styles (FAA background, dark overlay, hero image, badges)
• docs/gallery.md — new screenshots page with all device groups (iPhone, Landscape, Android, iPad)
• docs/index.md — switched to layout: home, removed screenshots section, added gallery link
• docs/screenshots/FAA.png + website-image.png — new image assets
• .gitignore — added .superpowers/ entry

Test Plan

• Verify homepage hero — FAA chart visible as dimmed background, app screenshot centered, white title/tagline readable
• Verify hero appears on all pages (release-notes, privacy, gallery)
• Verify App Store and Google Play badges appear on homepage
• Verify "View screenshots →" link navigates to /VatView/gallery
• Verify gallery page shows all four device groups
• Verify /VatView/privacy URL unchanged

Notes

home-custom.css is loaded via head-custom.html (Cayman's extension point) so it applies globally. The home.html layout only controls the hero structure on the homepage — other pages get the FAA background header via CSS without the hero image/badges.