Add fabro-static::EnvVars as the shared registry for fixed environment variable names and migrate env reads, clap env bindings, and subprocess/test allowlists to use it.
Add clippy bans for raw std::env lookup APIs so future dynamic env facades must be documented explicitly.
Implement the web-first install experience across the server, CLI, API spec,
web app, and packaged SPA assets.
This also removes test-side process env mutation by pushing env-dependent
decision points behind explicit helpers and test wiring.
