Update logo and favicon

[dpshade]

Replace the current logo and favicon with a new design that better represents Argo as a Permaweb navigation launcher.

Tasks:

• Design or source new logo
• Create favicon in multiple sizes (16x16, 32x32, 192x192, 512x512)
• Update logo in header/navbar
• Update favicon files and references in index.html
• Ensure logo works well in both light and dark modes