BUG: Weak random number generation for MIME boundaries

[andrinoff]

Describe the bug
In pgp/yubikey.go line 96, MIME boundary uses only time.Now().Unix() which is predictable and not cryptographically secure. While crypto/rand is used in sender.go, yubikey.go has weaker fallback.

To reproduce
Examine pgp/yubikey.go line 96:

boundary := fmt.Sprintf("----=_Part_%d", time.Now().Unix())

Compare to sender/sender.go lines 81, 283, 487 which use crypto/rand properly.

Expected behavior
Use crypto/rand for MIME boundaries to prevent prediction attacks.

Additional context

• Good first issue
• Low severity (MIME boundary prediction has limited attack surface)
• Best practice: always use crypto/rand for boundaries
• sender/sender.go shows correct pattern

Suggested fix:

buf := make([]byte, 16)
if _, err := rand.Read(buf); err == nil {
    boundary = fmt.Sprintf("----=_Part_%x", buf)
} else {
    // fallback
    boundary = fmt.Sprintf("----=_Part_%d", time.Now().UnixNano())
}

OS
All platforms