Address code review: fix imports, salt hashing, conditional delivery message

- Move asyncio import to module level in gmail_service.py
- Move datetime import to module level in providers.py
- Use hashlib.sha256 for per-user salt generation (no truncation risk)
- Make delivery method message conditional in AddMailAccountModal

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/de3ef930-a980-4958-8a9d-a2c802918e81

backend/app/api/v1/endpoints/providers.py

@@ -1,4 +1,5 @@
 """Provider presets and Gmail credential management endpoints"""
+from datetime import datetime
 from typing import List
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -197,14 +198,12 @@ async def save_gmail_credential(
         existing.encrypted_access_token = encrypted_access
         existing.encrypted_refresh_token = encrypted_refresh
         existing.is_valid = True
-        from datetime import datetime
         existing.last_verified_at = datetime.utcnow()
         await db.commit()
         await db.refresh(existing)
         return existing
     else:
         # Create new
-        from datetime import datetime
         credential = GmailCredential(
             user_id=current_user.id,
             gmail_email=credential_in.gmail_email,