Add Vietnamese translation functionality with Helsinki-NLP/opus-mt-en-vi model

- Created vi/ module with VietnameseTranslator class
- Added UI checkbox for Vietnamese translation toggle
- Integrated translation into SFT and RAG processing workflows
- Updated Dockerfile to download model during build
- Added comprehensive logging and error handling
- Updated requirements.txt with transformers and torch dependencies

vi/processing.py

@@ -92,4 +92,4 @@ def log_translation_stats(stats: Dict[str, Any], translated_count: int) -> None:
         translated_count: Number of items translated
     """
     stats["vietnamese_translated"] = translated_count
-    logger.info(f"Vietnamese translation completed: {translated_count} items translated")
+    logger.info(f"Vietnamese translation completed: {translated_count} items translated")