Add favicon kit and OG image (V0 polish)

[yyq1025]

Tracking favicon + social card follow-up. Current state: only a single SVG favicon (BrandMark) is configured. Works in modern browsers but missing:

Missing files

• `packages/website/public/favicon.ico` — currently Astro default, needs replacement (legacy fallback / accessed directly at `/favicon.ico`)
• `packages/website/public/apple-touch-icon.png` (180×180) — iOS Safari "Add to Home Screen" icon
• `packages/website/public/og.png` (1200×630) — Open Graph card for social sharing (HN, X, iMessage, Discord, Slack)
• `packages/website/public/site.webmanifest` — Android "Install to Home Screen" PWA support (optional V0)

Easiest path

1. Upload `packages/website/public/favicon.svg` to https://realfavicongenerator.net/ — generates full kit (multi-size .ico, apple-touch-icon, Android icons)
2. Drop generated files into `packages/website/public/`
3. Replace existing `<link rel="icon">` block in `packages/website/src/layouts/BaseLayout.astro` with their generated `` tags

OG image needs separate design

1200×630 horizontal card with:

• BrandMark + `sidecode` wordmark
• Tagline: "By your side. Code anywhere with Claude."
• Paper palette (#F2EFE4 bg, #EE5722 accent)

Options:

• Static export from Figma
• Dynamic generation with @vercel/og at build time

When to do

V0.5+ polish pass — once landing is otherwise ship-ready. Not blocking V0.