feat(rebrand): replace T3 production icon with BCode mark

Rebuild assets/prod/ from the new bcode.png source via ImageMagick
(1024 PNGs, multi-res ICOs, 180/32/16 web favicons) and rewrite
logo.svg from a potrace vectorization of the same source. Rename
t3-black-*/black-* files to bcode-* and update the paths in
scripts/lib/brand-assets.ts.

Dev assets (assets/dev/, assets/nightly/) left untouched — separate
pass.

Author: Berkay2002