Add i18n (internationalization) support

[wassertim]

Summary

The app currently has no internationalization support. All UI strings are hardcoded in English. Add lightweight i18n to support multiple languages.

Proposed Approach

Locale resolution (priority order):

1. Query param override (?lang=fr)
2. Accept-Language request header
3. Default fallback (en)

Translation system:

• Simple JSON key-value files per locale (e.g. locales/en.json, locales/fr.json)
• Thin t(key, locale) helper function for string lookup
• No heavy frameworks — keep it minimal and aligned with the project's KISS principle

Integration:

• Read locale in Hono middleware, attach to context
• Use t() in JSX components for all user-facing strings
• Extract existing hardcoded English strings into locales/en.json

Relevant Standards

• BCP 47 for language tags (en-US, fr-FR)
• Accept-Language header (RFC 9110) for browser negotiation
• ICU MessageFormat for plurals/interpolation if needed later

Tasks

• Create locales/en.json with all existing UI strings
• Implement t(key, locale) lookup helper
• Add Hono middleware to resolve locale from request
• Update JSX components to use t() instead of hardcoded strings
• Add at least one additional locale as a proof of concept