Add credit top-up UI and proxy route

[pranavpatilsce]

Summary

Adds a credit top-up flow to the reef UI so users can add LLM credits without leaving the chat.

What changed

• Header button — persistent + credits button in the top bar with a dropdown for $5/$10/$25/$50 top-ups. Always accessible, so users can add credits before starting big tasks.
• Inline widget — appears in the chat stream when a task finishes with "No credits available", with the same amount buttons right where the error shows up.
• Proxy route — POST /ui/api/topup-credits forwards the request to vers-landing's /api/reef/topup-credits endpoint using the reef auth token. The reef server already has VERS_AUTH_TOKEN in its env.

How it works

1. User clicks an amount button (header dropdown or inline widget)
2. Reef UI calls /ui/api/topup-credits (reef server)
3. Reef server proxies to ${VERS_LANDING_URL}/api/reef/topup-credits with the auth token
4. vers-landing verifies the token, checks for a Stripe card on file, charges it off-session, then calls the Chelsea admin API to grant credits to the team pool
5. UI shows success/failure feedback

New env var

• VERS_LANDING_URL — defaults to https://vers.sh, set on the reef VM to point to the vers-landing instance

Cross-repo context

Depends on vers-landing feat/fleet-credits branch which adds the /api/reef/topup-credits endpoint, the DB migration for llm_proxy_key_id, and the deploy route changes to store the LLM proxy key ID at exchange time.