Polish/browser fallback and install docs

[0xBreadguy]

No description provided.

[deva-arun]

The browser fallback behavior is useful, but src/auth/loopback.ts should not write directly to process.stderr. That layer should stay transport/auth-focused and not own terminal UX.

We already have command-level output plumbing via makeBrowserOpener() / injected stderr, which keeps JSON/terse output predictable and makes tests less global-stateful. Can we move fallback URL rendering into the makeBrowserOpener() callback and have openSystemBrowser() return false on open failure instead?

Also please update the test to assert against injected stderr and avoid monkey-patching global process.stderr.

Finally, can you rebase your changes as a single commit off main?

[deva-arun]

good idea, can we make this cross-platform guidance?

  error "Node.js >= $required_node_major is required, but you have Node.js $(node -v 2>/dev/null || echo unknown).

  Install Node.js 22, then rerun the installer.

  Options:
    nvm:      nvm install 22
    Homebrew: brew install node
    Ubuntu:   curl -fsSL https://deb.nodesource.com/setup_22.x | sudo -E bash - && sudo apt-get install -y nodejs"

[deva-arun]

see my comment on the PR

[deva-arun]

Can you replace the existing uninstall guidance in # Logout and Uninstall with this? Also change the instructions to pipe to bash ( | bash)