i18n scaffold: English only (no machine-translated locales)

## Goal
Add an i18n scaffold so UI/server user-facing strings are keyed. **Ship `en` only** from strings we already have. Real humans do other languages later — **no machine-translated locales**.

## Scope
- Message catalog / resource mechanism appropriate for KMP + Keel pack (document choice in DEVELOPING)
- Extract existing English strings into `en`
- Include default-calendar template key: `{name}'s Calendar` (see signup default-calendar issue)
- Clear contribution path for human translators (no auto-translate in CI)

## Out of scope
- Machine translation / LLM-filled locale files
- Shipping `es`/`de`/… until humans provide them

## Acceptance
- [ ] `en` catalog covers current user-facing strings (or tracked gaps listed)
- [ ] App runs with `en` as default/fallback
- [ ] Docs: how to add a human locale later