Add bb setup browser flow

[shuyhere]

Fixes #78

Summary

• add a first-class bb setup browser command
• detect whether a compatible local Chrome / Chromium / Edge executable is available
• print the current-shell BB_BROWSER export and optionally persist it to the user's shell rc file
• reuse browser_fetch detection logic from bb-tools
• document the new setup flow in the README and tool docs

Why

We considered making browser installation/configuration happen implicitly during every npm install, but that is too heavy and surprising for a large optional runtime dependency.

An explicit setup flow is safer and clearer:

• it tells the user whether browser_fetch can work on this machine
• it shows how to configure BB_BROWSER
• it can persist that config for future shells
• it gives platform-specific install guidance when no browser is found

Command examples

bb setup browser
bb setup browser --persist

Testing

• cargo test -p bb-cli setup -- --nocapture
• cargo test -p bb-tools browser_fetch -- --nocapture
• cargo run -q -p bb-cli -- setup browser --help
• cargo run -q -p bb-cli -- setup browser