v1.0.0 — Postbox "Monterail Dark" for Betterbird 140

First complete release. A pure userChrome/userContent CSS recreation of
Postbox's Monterail Dark look on Betterbird 140.12.0esr-bb24.

Themes the thread pane, folder pane, toolbar, tabs, message header, composer,
status bar, and the email body. All design values are --pb-* tokens in
chrome/postbird/config.css; every rule is mapped in docs/selector-map.md for
cheap repair after ESR bumps.

Install: clone → ./scripts/deploy.ps1 → enable
toolkit.legacyUserProfileCustomizations.stylesheets, switch to Cards view,
restart. See the README and CHANGELOG.md for details and known limitations.