Zoom the bandmap with the mouse wheel and add a Centre on VFO toggle

The wheel steps along the span list and keeps the frequency under the
pointer in place. With Centre on VFO off, the slice stays put until the
receiver nears an edge. The span and the toggle are saved in the settings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/Nonemm.App.Tests/SettingsTests.cs

@@ -93,6 +93,17 @@ public class SettingsTests : IDisposable
         Assert.Equal("F1 CQ,CQ", settings.CwMessageFile);
     }
 
+    [Fact]
+    public void TheBandmapViewIsStoredAndReadBack()
+    {
+        new Settings { BandmapSpanHertz = 0, BandmapCentred = true }.Save(path);
+
+        Settings read = Settings.Load(path);
+
+        Assert.Equal(0, read.BandmapSpanHertz);
+        Assert.True(read.BandmapCentred);
+    }
+
     /// An operator's windows and colours survive a trip through the file, and a
     /// file written before operators existed reads as a station with none.
     [Fact]