Add a string or a locale

Every user-facing string is a key in packages/i18n/messages/<locale>.json (en.json and es.json today). .claude/rules/i18n.md has the rules.

A string

1. Pick the namespace (top-level key) the surface uses: auth, settings, dashboard, email, notification, sms, errors, mobile, … and put the key next to its siblings. Reuse common.* for shared words (Save, Cancel).
2. Add it to every catalog, at the same path, in the same change. Write real translations; if you can't, say so in the pull request rather than copying English.
3. ICU for anything variable: {name}, {count, plural, one {# item} other {# items}}. Never concatenate translated fragments. Arguments aren't type-checked (the catalog is imported as JSON), so pass exactly the ones the message names.
4. Use it: useTranslations("<ns>") (web client, and use-intl on mobile), getTranslations from next-intl/server (web server components), the translator in apps/notifications for emails, push and SMS. Dates, numbers and money through useFormatter() / getFormatter().
5. Keys the code checks for: an error code needs errors.<CODE>; an event workspace.audit.events.<domain>.<fact>.v<N> (nested, like todo.created.v1); an API-key scope workspace.apiKeys.scopes.<scope>; a notification category notificationSettings.categories.<name>.

A locale

1. Copy packages/i18n/messages/en.json to <code>.json and translate it.
2. Add the code to locales and to catalogs in packages/i18n/src/index.ts (the satisfies clause makes a missing key a type error).
3. Right-to-left: add it to RTL in apps/web/src/app/layout.tsx.
4. The language switcher (apps/web/src/modules/shell/components/language-switcher.tsx) and mobile settings list locales, so they pick it up.

Done when

• bun run check-types passes (web keys are checked against English).
• bun run --filter @repo/i18n test passes (every catalog complete; codes, events, categories and scopes all have their keys).
• bun run --filter @repo/email test if an email changed.
• The i18n-checker agent reports nothing blocking.