Upgrade blueprint config, overlay script, and files-editor textareas with a
reusable vanilla-JS editor. Textarea stays as value carrier so form POST and
files-overlay.js fetch paths are untouched. Seed cvar vocabulary from the
existing l4d2-server-cvar-reference.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
