Add start game time feature to world generation; update API and UI to support custom start dates for in-world calendar

src/TheLivingWorld.Api/Simulation/WorldSimulation.cs

@@ -70,9 +70,9 @@ public sealed class WorldSimulation : IDisposable
         return simulation;
     }
 
-    public static WorldClockDto DefaultClock() => new()
+    public static WorldClockDto DefaultClock(DateTime? startGameTime = null) => new()
     {
-        GameTime = GameTime.DefaultStart,
+        GameTime = GameTime.ResolveStart(startGameTime),
         TimeScale = GameTime.MinTimeScale,
         Paused = false,
     };