improvement: some files replaced, lots of them i shall say. AGENT.MD file created. some rules and skills added for cursor agent.

.cursor/rules/api-errors-i18n.mdc

@@ -0,0 +1,28 @@
+---
+description: Error codes backend ↔ frontend and i18n message keys
+globs: backend/src/common/errors/**,frontend/src/components/shared/formatApiError.ts,frontend/messages/**
+alwaysApply: false
+---
+
+# API errors & translations
+
+## Adding a new error
+
+1. Add code to `backend/src/common/errors/error-codes.ts`
+2. Throw via `AppException` (or validation DTO with that code)
+3. Add matching key under `errors` in **all three** message files:
+   - `frontend/messages/en.json`
+   - `frontend/messages/fa.json`
+   - `frontend/messages/nl.json`
+4. Frontend catch: `getUserFacingError(err, tErrors, t('fallbackKey'))`
+
+## Validation field errors
+
+Backend returns `{ success: false, error: { code, details: [{ field, code }] } }`.
+
+Frontend maps `details[].code` through the `errors` namespace.
+
+## Do not
+
+- Show raw `error.message` or stack traces to users.
+- Add English-only strings inline in components.