Add TUI auth profile login flow

[shiny-code-bot]

Why

Codex Lab is moving toward daily dogfooding with multiple auth accounts. The existing /login picker could switch among previously-created auth profiles, but adding another account still required leaving the TUI and running a shell command. That made the first multi-auth flow feel unfinished and easy to forget during normal use.

What Changed

• Makes the /login picker’s Add login... row actionable by pre-filling /login add in the composer.
• Adds /login add <profile> to validate a new profile name, reject ambiguous names such as default, switch to that profile’s auth_home, and start the ChatGPT browser login flow.
• Tracks the pending profile login by login_id, surfaces login failure, and records auth-profile metadata only after the post-login AccountUpdated notification confirms the app server has reloaded auth.
• Clears stale pending profile-login state before later auth-profile switches so unrelated account updates cannot create phantom profile metadata.

Verification

• cargo test -p codex-tui login_slash_command
• cargo check -p codex-tui