Add version notifications for new releases

[chriswritescode-dev]

Summary

• Adds version check endpoint that queries GitHub Releases API to detect new versions
• Shows toast notification on app load when update is available
• Displays current version in GeneralSettings with link to new release
• Includes PR Add opencode-manager version to health endpoint #36 changes (adds opencodeManagerVersion to health endpoint)

Changes

• Backend: Adds /api/health/version endpoint with GitHub API integration (1-hour cache)
• Backend: Fixes hardcoded version in root endpoint
• Backend: Moves health routes under auth
• Frontend: Adds useVersionCheck hook and VersionNotifier component
• Frontend: Shows version in GeneralSettings header

Testing

• TypeScript typecheck: passed
• ESLint: passed
• Tests: 337 passed

Closes #36