033: fix preview_translation route + MultiSelect label regression

1. preview_translation route: missing await on async preview_rows()
   - preview_rows() is async def, called without await
   - returned coroutine object instead of result -> 'coroutine not iterable' error

2. MultiSelect.svelte: opt.label -> opt.name
   - option type is {code, name} but template used {opt.label}
   - rendered empty spans instead of language names

backend/src/api/routes/translate/_preview_routes.py

@@ -43,7 +43,7 @@ async def preview_translation(
         payload = PreviewRequest()
     try:
         preview_service = TranslationPreview(db, config_manager, current_user.username)
-        result = preview_service.preview_rows(
+        result = await preview_service.preview_rows(
             job_id=job_id,
             sample_size=payload.sample_size,
             prompt_template=payload.prompt_template,