Refactor FTL time tracking code to fix a UI bug (#26538)

Refactor FTL time tracking code to fix a UI bug (#26538)

The FTL UI on the shuttle console would reset the FTL progress bar every time you open it. This is because the server only sends "time until completion", not a start/end time. The FTL code now uses a separate start/end time so the exact same progress bar can be preserved.

For convenience, I made a StartEndTime record struct that stores the actual tuple. This is now used by the code and has some helpers.