Enhancement: Display timezone information in snapshots output

The `snapshots` command now displays which timezone is being used to show
timestamps. Since snapshots can be created in different timezones but are
always displayed in the local timezone, a footer line is now shown indicating
the timezone used for display (e.g., "Timestamps shown in CET timezone").
This helps prevent confusion when comparing snapshots in a multi-user
environment.

https://github.com/restic/restic/pull/5588