i18n: extract hardcoded strings and add translations

[JohnRDOrazio]

Summary

We have next-intl installed and configured, and an English message file (messages/en.json) with a basic set of keys. However, the vast majority of frontend strings are still hardcoded in components rather than using useTranslations() / t() calls.

Current state

• next-intl is set up with locale routing (app/[locale]/)
• Only messages/en.json exists — no other languages
• The message file covers a small subset of keys (common, auth, nav, ontology, class, property, collab, git, errors)
• Most component strings are hardcoded in English and do not use the translation functions

Work needed

1. Audit components for hardcoded user-facing strings (buttons, labels, headings, tooltips, error messages, placeholders, etc.)
2. Extract strings into messages/en.json under appropriate namespaces, and replace hardcoded text with t() calls
3. Add at least one additional locale to validate the full i18n pipeline end-to-end
4. Add a language switcher UI component

Acceptance criteria

• No user-facing strings are hardcoded in components
• All strings are organized in messages/en.json with clear namespaces
• At least one additional language file exists (e.g. messages/it.json)
• Language switcher is available in the UI
• next-intl locale detection and routing work correctly