Function sanitizeAttributes

Sanitize a map of HTML attributes, returning a new map containing only entries that pass all safety checks.

For each attribute, this function:

1. Drops attributes that fail isSafeAttribute (event handlers, unknown names)
2. Drops URL-bearing attributes whose values fail isDangerousUrl
3. Sanitizes style values via sanitizeStyleValue, dropping them entirely if the result is empty
4. Passes all other safe attributes through unchanged