Add favicon assets

[Legendaryabysss]

Fixes #1.

Summary

• Copied the favicon and platform icon assets from bigearth/bitbox.earth/src/assets.
• Added the Apple touch icons, PNG favicons, .ico, web manifest, and Microsoft tile metadata to the Jekyll layout.
• Kept all asset links behind Jekyll's relative_url helper so the site works correctly under the /bitblog base path.
• Adjusted the copied manifest and browserconfig paths to point at sibling files in /assets.

Verification

• Checked that every asset referenced by _layouts/default.html exists locally.
• Checked that every manifest.json and browserconfig.xml icon reference resolves to a copied asset.
• Checked representative icon dimensions with file.

I attempted a full bundle exec jekyll build, but the repo's older github-pages bundle resolves to ffi 1.9.21, which fails to build against this macOS/Ruby toolchain. The static asset/path verification above passes.

[Legendaryabysss]

Transparency note: this PR was prepared with AI assistance using Codex. I kept the change scoped, reviewed the diff, and verified the generated favicon assets/head tags locally before opening it.