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

.cursor/skills/api-errors/SKILL.md

@@ -0,0 +1,40 @@
+---
+name: dyolink-api-errors
+description: Adds or migrates Dyolink API error codes with frontend translations. Use when adding backend validation errors, permission errors, or migrating catch blocks to getUserFacingError.
+---
+
+# Dyolink API errors
+
+## Backend
+
+1. Add to `ErrorCode` in `backend/src/common/errors/error-codes.ts`.
+2. Throw with `AppException`:
+
+```typescript
+throw new AppException(ErrorCode.MY_CODE, HttpStatus.BAD_REQUEST, [
+  { field: 'email', code: ErrorCode.VALIDATION_EMAIL_INVALID },
+]);
+```
+
+3. DTOs: `@IsEmail({}, { message: ErrorCode.VALIDATION_EMAIL_INVALID })`
+
+## Frontend
+
+1. Add key under `"errors"` in `en.json`, `fa.json`, `nl.json` (key = error code string).
+2. In components:
+
+```typescript
+const tErrors = useTranslations('errors');
+// ...
+catch (err: unknown) {
+  toast.showError(getUserFacingError(err, tErrors, t('fallbackKey')));
+}
+```
+
+3. Do not use `err.message` or `(err as Error).message` for user display.
+
+## Axios shape
+
+Parsed in `lib/api/client` — expects `{ success: false, error: { code, details? } }`.
+
+See rule: `.cursor/rules/api-errors-i18n.mdc`