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/core/security.py

@@ -1,6 +1,7 @@
 """
 Security utilities for encryption, hashing, and token generation.
 """
+import hashlib
 import secrets
 from datetime import datetime, timedelta
 from typing import Optional, Dict, Any
@@ -81,10 +82,9 @@ class CredentialEncryption:
         if key is None:
             key = settings.ENCRYPTION_KEY
         
-        # Generate salt - in production, this should be unique per user
+        # Generate salt - unique per user for enhanced security
         if user_id is not None:
-            # Per-user salt for production
-            salt = f'pop3fwd_usr_{user_id}'.encode('utf-8')[:16].ljust(16, b'\x00')
+            salt = hashlib.sha256(f'pop3fwd_usr_{user_id}'.encode()).digest()[:16]
         else:
             # Default salt for system-wide operations (use with caution)
             salt = b'pop3_forwarder_0'