feat: add BITBOX favicon assets

[Adamchaua]

Summary

• add favicon and touch-icon assets from bitbox.earth
• wire favicon, manifest, and Microsoft tile tags into the default layout

Verification

• ran git diff --check
• confirmed the layout references the copied assets with relative_url

References #1

[Adamchaua]

Hi maintainers — this is a small asset-focused PR adding BITBOX favicon assets. It should not affect runtime logic. If you want the assets exported in a different size set, path, or naming scheme to match the existing brand pipeline, I can update it.