Add a cross-provider Review Center and improve credential handling.
Backend integrations fetch and normalize PRs from GitHub, GitLab,
Gitea, and Azure DevOps with improved timeouts and parsing. Credentials
now use a global lock and support secret-tool on Linux to avoid races.
- Normalize review data across providers (GitHub/GitLab/Gitea/Azure)
- Serialize credential access with OnceLock and use secret-tool on Linux
- Add frontend ReviewCenter component and related UI updates