[P3] Add a localization layer for the user interface #364

Priority

P3 - the app supports non-English game imports but forces English UI, language metadata, and number formatting.

Finding

index.html hardcodes lang=en, UI strings are embedded in components, and some number formatting forces en-US. Browser language preferences do not affect the interface.

Acceptance criteria

• Route user-facing strings through a localization layer.
• Update documentElement.lang for the selected locale.
• Format numbers with the selected locale.
• Provide and test English plus at least the existing Korean-user fallback path.