refactor(fit-app): address second code-quality pass on #930

- WeeklyRing.UpdateArc: assign the new PathGeometry directly to ArcPath.Data
  instead of via a throwaway local (the Path owns it; the previous geometry is
  still disposed before replacement) — clears the "IDisposable not disposed" flag.
- Shell.StartupLoadable.Begin: drop the defensive generic catch around
  Task.Delay. A constant delay with no cancellation token cannot throw; the
  splash-dismissal guarantee is the TryEnqueue/Complete fallback, which stays.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Jen-Uno

studio/fit-app/FitBeginnerApp/Presentation/Shell.xaml.cs

@@ -39,17 +39,10 @@ private sealed class StartupLoadable : ILoadable
 
         public async void Begin(DispatcherQueue dispatcher, TimeSpan delay)
         {
-            try
-            {
-                await Task.Delay(delay);
-            }
-            catch
-            {
-                // Swallow: the splash must still be dismissed below.
-            }
+            await Task.Delay(delay);
 
-            // Prefer the UI thread; if enqueue fails (dispatcher shutting down), complete inline
-            // so IsExecuting is never left stuck true.
+            // Prefer the UI thread; if enqueue fails (dispatcher shutting down), complete inline so
+            // IsExecuting is never left stuck true — this is what guarantees the splash is dismissed.
             if (!dispatcher.TryEnqueue(Complete))
             {
                 Complete();

studio/fit-app/FitBeginnerApp/Presentation/WeeklyRing.xaml.cs

@@ -161,12 +161,11 @@ private void UpdateArc()
             IsLargeArc = sweep > 180,
         });
 
-        var geometry = new PathGeometry();
-        geometry.Figures.Add(figure);
-        // Dispose the geometry from the previous update before it is replaced (UpdateArc runs on
-        // every progress tick during the entrance animation, so a fresh geometry is built each time).
+        // Dispose the geometry from the previous update before replacing it (UpdateArc runs on every
+        // progress tick during the entrance animation, so a fresh geometry is built each time). The new
+        // geometry is owned by ArcPath.Data — assign it directly rather than via a throwaway local.
         (ArcPath.Data as IDisposable)?.Dispose();
-        ArcPath.Data = geometry;
+        ArcPath.Data = new PathGeometry { Figures = { figure } };
     }
 
     private static Point PointOnCircle(double angleDegrees)