Small fixes to time zones, connections, download sign-in errors and file backups

Several small behaviour changes:

• Time zone: Claude Code now also looks at the TZ environment variable when deciding whether the computer runs on UTC time. Before, it relied only on the system's reported time zone.
• Connections: one kind of network connection is now opened on its own instead of through a shared connection pool.
• Downloads: a download that fails with a 401 error, which means not authorised, is now marked as the sign-in token being refused.
• File backups: when Claude Code saves a backup copy of a file before changing it, the copy now keeps the original file's last-opened and last-modified dates.