BUG: Attachment filename not sanitized for filesystem

[andrinoff]

Describe the bug: Attachment filenames used directly when saving to disk. Could contain path traversal characters like "../" or invalid filesystem chars.

To reproduce:

1. Receive email with attachment named "../../evil.txt"
2. Save attachment
3. May write outside intended directory

Expected behavior: Sanitize filename:

• Strip path separators (/ )
• Remove invalid chars for current OS
• Limit length to filesystem max
• Handle collisions with (1), (2) suffix

Additional context:

• File: view/html.go or attachment handling code
• Use filepath.Base() and validate chars
• Severity: Medium - path traversal risk
• Complexity: Easy - filename sanitization