fix: 🐛 guard against negative nc:lock-timeout when computing lockTimeOut

When files_lock is configured with infinite timeout (-1 minutes), the server
sends nc:lock-timeout=-60. Without the guard, lockTimeOut would be set to
lockTime minus 60 seconds — a date in the past — causing clients to display
the lock as already expired.

Only compute lockTimeOut when the value is positive; leave it nil for infinite
locks (zero or negative sentinel values).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: James Manuel <moodyjmz@users.noreply.github.com>

Author: moodyjmz

Sources/NextcloudKit/Models/NKDataFileXML.swift

@@ -472,7 +472,7 @@ public class NKDataFileXML: NSObject {
             if let lockTime = propstat["d:prop", "nc:lock-time"].int {
                 file.lockTime = Date(timeIntervalSince1970: TimeInterval(lockTime))
             }
-            if let lockTimeOut = propstat["d:prop", "nc:lock-timeout"].int {
+            if let lockTimeOut = propstat["d:prop", "nc:lock-timeout"].int, lockTimeOut > 0 {
                 file.lockTimeOut = file.lockTime?.addingTimeInterval(TimeInterval(lockTimeOut))
             }