✨ Implement Hangar platform support (fix HANGER typo)

[Nlkomaru]

Summary

Hangar (PaperMC's plugin repository) is advertised in documentation but is not functional.

Current state

• RepositoryType.kt line 23: Enum value is misspelled as HANGER instead of HANGAR
• DownloaderRepositoryImpl.kt lines 237-239: Hangar downloads return null
• DownloaderRepositoryImpl.kt lines 119-122: Version retrieval returns generic "unknown" version data
• URL parsing for Hangar exists (PluginMetadataManagerImpl.kt lines 82-86) but leads to dead ends

What needs to be done

1. Fix typo: Rename HANGER → HANGAR in RepositoryType.kt (this is a breaking change for any existing configs using the string)
2. Implement HangarDownloader: Create a downloader that uses the Hangar API
3. Wire into all paths: Ensure Hangar works in:
   • getLatestVersion()
   • getVersionByName()
   • getAllVersions()
   • downloadByVersion()
4. Add tests: At least contract tests for version resolution
5. Update docs: Confirm Hangar support in documentation

Impact

• Documentation promises Hangar support that doesn't work
• Users trying to install from Hangar will get confusing errors