[codex] add referral invites to /usage

[jayp-oai]

Summary

• add a temporary client-owned referral invite flow under /usage
• call the existing ChatGPT referral HTTP endpoints directly instead of adding app-server RPC/protocol endpoints
• reuse the embedded app-server auth session, pin user/workspace identity and confirmed reward terms, and hide the flow when a referral client is unavailable
• require email entry plus explicit confirmation, revalidate the offer immediately before send, and send the invite with a no-retry/no-redirect POST client

Scope

• 24 files, 1,203 additions, 26 deletions
• no new backend endpoint or public app-server API

Testing

• cargo check -p codex-login -p codex-chatgpt -p codex-tui
• just test -p codex-chatgpt (8 passed)
• just test -p codex-tui referrals (7 passed)
• just fix -p codex-login -p codex-chatgpt -p codex-tui
• just fmt
• git diff --check origin/main...HEAD
• Autoreview: clean, no accepted/actionable findings