Add BugTrail logo: extension icons, favicons and popup/options branding ...

SVG mark (brand/bugtrail.svg): a bug at the head of the trail of steps
it walked, Tokyo Night palette. PNG icons 16/32/48/128 are rendered
from it (verify-logo.mjs) and shipped in the extension manifest;
web panel gets an SVG favicon; popup and options headers show the mark
instead of the generic bug glyph.

Co-Authored-By: Claude Code <noreply@anthropic.com>