branding: lantern icon for favicon + nav logoVerified

Swap the cauldron for a lantern (a light in the cave) with a </> code glyph.
favicon.svg is the self-contained mark (dark circle + white lantern). The nav
logo is a currentColor, no-background version (stroke + glyph) so it takes the
brand accent and adapts to any theme.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>