- Add repository URL validation for GitHub, GitLab, Bitbucket, and custom hosts
- Add git provider layer (listDirectory, downloadRawFile) for all providers
- Wire githubJsonService and scriptDownloader to use provider; sync/download from any supported source
- Update GeneralSettingsModal placeholder and help text; .env.example and env schema for GITLAB_TOKEN, BITBUCKET_APP_PASSWORD
Closes#406