feat: language dropdowns on versions and on the game page

- Each ready version gets a "Languages" dropdown next to Play: the original language
  first, then translations with their completion; picking one starts the version in
  that language
- The game header gets the same dropdown, aggregated over published, ready versions.
  Each language points at the version that has it best (most complete, the main
  version on a tie); an original language counts as complete
- The old row of language tags linking to the translation editor is replaced
- Backend: versions carry source_name (the name of the original language)
- pages/languages.ts holds the aggregation logic with tests; LanguageMenu closes on
  outside click and Escape

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/app/schemas.py

@@ -82,6 +82,7 @@ class VersionOut(ORM):
     kind: str = "original"
     # from the version's game.json (filled by the game detail endpoint)
     source_lang: str | None = None
+    source_name: str | None = None
     languages: list[dict[str, Any]] = []