docs(i18n): add conventions guide and wire docs navigation

docs/product-engineering/i18n-conventions.md

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+# i18n Conventions
+
+## Scope
+
+This document defines translation conventions for both apps in MVP0+.
+
+- Public app i18n: `next-intl` message namespaces and route-level usage
+- Admin app i18n: JSON dictionaries + runtime resolver/provider
+- Shared locale contract: `@cms/i18n` (`de`, `en`, `es`, `fr`; default `en`)
+
+## Locale Policy
+
+- Source of truth: `packages/i18n/src/index.ts`
+- Current enabled locales are code-driven and shared across web/admin.
+- Admin-managed locale toggles are planned for a later MVP.
+
+## Key Naming Conventions
+
+- Use `camelCase` for keys.
+- Group by domain namespace (not by component filename).
+- Keep keys stable; update values, not key names, during copy edits.
+
+### Public app namespaces
+
+- `Layout.*`
+- `Home.*`
+- `LanguageSwitcher.*`
+- Page-specific namespaces, e.g. `About.*`, `Contact.*`
+- Metadata namespace: `Seo.*`
+
+### Admin app namespaces
+
+- `common.*`
+- `auth.*`
+- `dashboard.*`
+- `settings.*`
+
+## Message Structure
+
+- Keep messages as nested JSON objects.
+- Avoid very deep nesting (prefer 2-3 levels).
+- Keep punctuation in translation values, not code.
+- Avoid embedding HTML in message strings.
+
+## Fallback Rules
+
+- Unknown/invalid locale values fallback to default locale `en`.
+- Missing translation key behavior:
+  - Admin: `translateMessage` returns provided fallback, else key.
+  - Public: ensure required keys exist in locale JSON; avoid runtime missing-key states.
+
+## Adding New Translation Keys
+
+1. Add key/value in `apps/*/src/messages/en.json`.
+2. Add equivalent key in `de/es/fr` JSON files.
+3. Use key via translator:
+   - Web: `useTranslations("Namespace")` or `getTranslations("Namespace")`
+   - Admin: `useAdminT()` or server-side `translateMessage(...)`
+4. Add/adjust tests for behavior where relevant.
+
+## Translation Workflow
+
+1. Author English source copy first.
+2. Add keys in all supported locales in same change.
+3. Keep semantic parity across locales.
+4. Run checks:
+   - `bun run check`
+   - `bun run typecheck`
+   - `bun run test`
+5. For route-level i18n behavior changes, run e2e smoke:
+   - `bunx playwright test --grep "i18n smoke"`
+
+## QA Checklist
+
+- Locale switch persists after refresh.
+- Page headings and navigation labels translate correctly.
+- Metadata (`Seo`) strings resolve per locale.
+- No missing-key placeholders visible in UI.
+
+## Related Files
+
+- `apps/web/src/i18n/request.ts`
+- `apps/web/src/i18n/routing.ts`
+- `apps/admin/src/i18n/server.ts`
+- `apps/admin/src/i18n/messages.ts`
+- `packages/i18n/src/index.ts`