fix: replace panic with fallback in ID generation (#213)

Replace panic calls in generateID() and generateThreadID() with
time-based fallback when crypto/rand.Read fails. This is an extremely
rare error case, but panicking is not the right behavior for ID
generation functions.

🤝 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/types.go

@@ -4,6 +4,7 @@ package mail
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"fmt"
 	"strings"
 	"time"
 )
@@ -142,19 +143,23 @@ func NewReplyMessage(from, to, subject, body string, original *Message) *Message
 }
 
 // generateID creates a random message ID.
+// Falls back to time-based ID if crypto/rand fails (extremely rare).
 func generateID() string {
 	b := make([]byte, 8)
 	if _, err := rand.Read(b); err != nil {
-		panic("crypto/rand.Read failed: " + err.Error())
+		// Fallback to time-based ID instead of panicking
+		return fmt.Sprintf("msg-%x", time.Now().UnixNano())
 	}
 	return "msg-" + hex.EncodeToString(b)
 }
 
 // generateThreadID creates a random thread ID.
+// Falls back to time-based ID if crypto/rand fails (extremely rare).
 func generateThreadID() string {
 	b := make([]byte, 6)
 	if _, err := rand.Read(b); err != nil {
-		panic("crypto/rand.Read failed: " + err.Error())
+		// Fallback to time-based ID instead of panicking
+		return fmt.Sprintf("thread-%x", time.Now().UnixNano())
 	}
 	return "thread-" + hex.EncodeToString(b)
 }