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>

frontend/src/api.ts

@@ -32,6 +32,7 @@ export interface Version {
   visibility: "draft" | "published";
   kind: "original" | "translation" | "mod";
   source_lang: string | null;
+  source_name?: string | null;
   languages: { code: string; name: string; progress: number }[];
   progress: string;
   size_bytes: number;