Translation Manager: dates shown in local time

Translation Manager: fixed backoffice dates (job nodes table, job list, history, versions) showing UTC instead of the user's local time. Dates were stored as UTC but came back from NPoco as Unspecified, so they serialised without an offset.
Added a UTC DateTime JSON converter on Translation Manager's own named JSON options (backoffice options + converter), so every Translation Manager API date gets a Z without touching Umbraco's or other packages' endpoints. This replaces per-field fixes in the mappers.
Standardised date writes on DateTime.UtcNow (repositories, job workflow, background items + health check)