added i18n for backend errors

src/errors.ts

@@ -0,0 +1,30 @@
+import { useFluent } from 'fluent-vue';
+import { ApiError } from './api/client';
+
+export function useErrorTranslator() {
+    const { $t } = useFluent();
+
+    function translateError(err: unknown): string {
+        if (err instanceof ApiError) {
+            // Convert "AUTH_INVALID_CREDENTIALS" -> "error-auth-invalid-credentials"
+            const key = `error-${err.code.toLowerCase().replace(/_/g, '-')}`;
+
+            const translated = $t(key);
+
+            // Fallback to the message provided by backend.
+            if (translated === key) {
+                return err.message;
+            }
+
+            return translated;
+        }
+
+        if (err instanceof Error) {
+            return err.message;
+        }
+
+        return $t('shared-error');
+    }
+
+    return { translateError };
+}