Guard AltitudeChartRenderer against uninitialized PlannerState

When no profile is loaded, PlannerState.AstroDark is default (0001-01-01),
causing DateTimeOffset arithmetic to overflow. Now returns early with just
the background fill when no data is available.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sebgod

src/TianWen.UI.Abstractions/AltitudeChartRenderer.cs

@@ -71,6 +71,13 @@ public static void Render<TSurface>(
         var w = (int)renderer.Width;
         var h = (int)renderer.Height;
 
+        // Guard: no data loaded yet (AstroDark is default = 0001-01-01)
+        if (state.AstroDark == default)
+        {
+            FillRect(renderer, 0, 0, w, h, BackgroundColor);
+            return;
+        }
+
         // --- Layout (proportional to renderer size) ---
         var xMargin     = Math.Max(60, w / 14);
         var yMarginTop  = Math.Max(36, h / 22);