fix: fall back to HTTP registration when Playwright loses workspace_id

[tianheil3]

Summary

• add the selectable registration execution mode and browser-based registration flow carried on this branch
• default registration back to the HTTP/curl_cffi flow because the Playwright path can stall before a workspace_id is resolved
• normalize any incoming playwright execution mode to curl_cffi and remove the Playwright option from the UI
• add regression tests covering the fallback behavior and the workspace_id recovery path
• include the 5SIM client changes already present on this branch

Problem

During browser/Playwright registration, the flow can pass email verification and about-you, then land on https://auth.openai.com/add-phone. Once it hits that phone-verification checkpoint, the pipeline no longer reaches the callback/access-token path that is used to recover workspace_id, so registration fails.

Fix

Keep the browser-related branch work in place, but force new registrations back onto the HTTP/curl_cffi flow by default. The backend now downgrades stale playwright execution mode requests to curl_cffi, and the frontend no longer offers Playwright as a selectable mode.

Testing

• D:\projects\codex-register\.venv\Scripts\python.exe -m pytest -q tests\test_browser_registration_flow.py (7 passed)
• D:\projects\codex-register\.venv\Scripts\python.exe -m pytest -q tests\test_workspace_cookie_parsing.py (2 passed)
• D:\projects\codex-register\.venv\Scripts\python.exe -m pytest -q tests\test_registration_post_create_login_flow.py (1 passed)