feat(issue-226): add shared backend-frontend key-map and locale contract

docs/ISSUE-226-SHARED-KEYMAP-LOCALE-CONTRACT.md

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+# ISSUE-226 — Shared key-map + locale-kontrakt (backend/frontend)
+
+## Source of truth
+- Single shared artifact: `shared/i18n/lobby.json`
+- Ownership is documented under `contract.ownership` in the same artifact.
+
+## Locale contract
+Defined under `contract.locale`:
+- default locale: `en`
+- supported locales: `en`, `da`
+- fallback rule: use default locale when requested locale is unsupported or a key translation is missing.
+
+## Shared backend→frontend key-map
+Defined under `contract.backend_to_frontend_error_keys`.
+
+Examples:
+- `session_not_found -> session_not_found`
+- `session_not_joinable -> join_failed`
+- `round_start_invalid_phase -> start_round_failed`
+
+This allows backend error codes to remain stable while frontend copy keys stay UX-oriented.
+
+## da/en example values
+From `shared/i18n/lobby.json`:
+- `frontend.errors.session_code_required.en = "Session code is required."`
+- `frontend.errors.session_code_required.da = "Sessionskoden er påkrævet."`
+
+## Verification
+- Backend: `python manage.py test lobby.tests.I18nResolverTests`
+- Frontend: `npm test -- --run tests/lobby-i18n.contract.test.ts`