First stable release of the Drift Website engine. Self-updates now come
from github.com/drift-creative-systems/drift-website (public, release
assets, 6-hour checks). Drift admin menu uses dashicons-layout and is
pinned to the top of the sidebar. Adds tools/build-zip.py for release zips.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>