Add word list edit mode with bulk delete and fix word deletion

Show checkboxes and batch actions only after entering edit mode, and delete related quiz records before removing words to avoid foreign key failures.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/services/word_service.py

@@ -4,7 +4,7 @@ from typing import Optional
 from fastapi import HTTPException
 from sqlalchemy.orm import Session
 
-from models import Word, User
+from models import QuizRecord, Word, User
 
 
 def utc_now_iso() -> str:
@@ -68,6 +68,10 @@ class WordService:
 
     def delete_word(self, db: Session, user: User, word_id: int) -> None:
         word = self.get_word(db, user, word_id)
+        db.query(QuizRecord).filter(QuizRecord.word_id == word.id).delete(
+            synchronize_session=False
+        )
+        db.flush()
         db.delete(word)
         db.commit()