Add explicit bb setup browser flow for browser_fetch dependencies

[shuyhere]

Summary

browser_fetch needs a first-class setup flow for its local browser dependency.

Today, users may hit a missing-browser error and still have to figure out Chrome/Chromium installation and BB_BROWSER configuration themselves.

Motivation

We considered making browser installation/configuration happen automatically during every npm install, but that has major downsides:

• large download / install side effects
• slower installs and CI friction
• platform-specific behavior
• surprising changes for users who never use browser_fetch
• downloaded browser builds may still not be compatible with the current browser_fetch launch mode

An explicit setup flow is safer and clearer.

Proposal

Add a dedicated command such as:

bb setup browser

Initial scope:

• detect whether a compatible Chrome / Chromium / Edge executable is available
• show the resolved executable path if found
• print the shell export needed for BB_BROWSER
• optionally persist that export to the user's shell rc file
• if nothing is found, print platform-specific install guidance and tell the user what to run next

Nice follow-ups

• TUI shortcut/help when browser_fetch fails due to missing browser
• optional bb doctor / dependency check flow
• optional guided installation helpers per platform