Fix demo spectrum at startup and frequency mismatch after STOP

- Remove FillDemoSpectrum from FormCreate: spectrum no longer shows
  fake signal data before the transceiver is connected.
- Add SaveStartupPreview to STOP path: startup.vfo_a now matches the
  device band cache so the display is consistent before and after
  clicking START on a fresh launch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

MainForm.pas

@@ -1059,7 +1059,6 @@ begin
   FSpecView.SpecRange      := FSpecRange;
   FSpecView.SpecGridStep   := FSpecGridStep;
   FSpecView.WfFrameInterval := FWaterfallFrameInterval;
-  FSpecView.FillDemoSpectrum;
 
   FNetwork := THPSDRNetwork.Create;
   FNetwork.OnDeviceFound := OnDeviceFound;
@@ -2587,8 +2586,9 @@ begin
     begin
       SaveCurrentBand;
       FSettings.SaveGlobal(FDevMAC, MakeGlobalSettings);
-      FSettings.Save;
     end;
+    FSettings.SaveStartupPreview(FVfoA, FVfoB, FSampleRate);
+    FSettings.Save;
     FSpecView.ResetSpectrumBuf;
     FSpecView.DrawSpectrum;
     PbSpectrum.Invalidate;