menuband landing: add uniform AC corner label
Mirrors the pattern from aesthetic-direct.html so the page is consistent
with the rest of AC's html-rendered paths. Uses Berkeley Mono in the
top-left, fading from --aqua-sky-top to transparent so it stays legible
over the music drift, and clicking returns the visitor to /
(or aesthetic.computer when viewing from another host).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>