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.

Content.Client/Shuttles/UI/MapScreen.xaml.cs

@@ -5,6 +5,7 @@ using Content.Shared.Shuttles.BUIStates;
 using Content.Shared.Shuttles.Components;
 using Content.Shared.Shuttles.Systems;
 using Content.Shared.Shuttles.UI.MapObjects;
+using Content.Shared.Timing;
 using Robust.Client.AutoGenerated;
 using Robust.Client.Graphics;
 using Robust.Client.UserInterface;
@@ -38,16 +39,11 @@ public sealed partial class MapScreen : BoxContainer
     private EntityUid? _shuttleEntity;
 
     private FTLState _state;
-    private float _ftlDuration;
+    private StartEndTime _ftlTime;
 
     private List<ShuttleBeaconObject> _beacons = new();
     private List<ShuttleExclusionObject> _exclusions = new();
 
-    /// <summary>
-    /// When the next FTL state change happens.
-    /// </summary>
-    private TimeSpan _nextFtlTime;
-
     private TimeSpan _nextPing;
     private TimeSpan _pingCooldown = TimeSpan.FromSeconds(3);
     private TimeSpan _nextMapDequeue;
@@ -114,8 +110,7 @@ public sealed partial class MapScreen : BoxContainer
         _beacons = state.Destinations;
         _exclusions = state.Exclusions;
         _state = state.FTLState;
-        _ftlDuration = state.FTLDuration;
-        _nextFtlTime = _timing.CurTime + TimeSpan.FromSeconds(_ftlDuration);
+        _ftlTime = state.FTLTime;
         MapRadar.InFtl = true;
         MapFTLState.Text = Loc.GetString($"shuttle-console-ftl-state-{_state.ToString()}");
 
@@ -511,20 +506,8 @@ public sealed partial class MapScreen : BoxContainer
             MapRebuildButton.Disabled = false;
         }
 
-        var ftlDiff = (float) (_nextFtlTime - _timing.CurTime).TotalSeconds;
-
-        float ftlRatio;
-
-        if (_ftlDuration.Equals(0f))
-        {
-            ftlRatio = 1f;
-        }
-        else
-        {
-            ftlRatio = Math.Clamp(1f - (ftlDiff / _ftlDuration), 0f, 1f);
-        }
-
-        FTLBar.Value = ftlRatio;
+        var progress = _ftlTime.ProgressAt(curTime);
+        FTLBar.Value = float.IsFinite(progress) ? progress : 1;
     }
 
     protected override void Draw(DrawingHandleScreen handle)