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>

src/Nonemm.App/Configuration/Settings.cs

@@ -53,6 +53,13 @@ public sealed record Settings
     /// How long a spot stays on the bandmap. N1MM's own default is 60.
     public int SpotTimeoutMinutes { get; init; } = 60;
 
+    /// The width of the slice the bandmap shows, in hertz. Zero is the whole band.
+    public int BandmapSpanHertz { get; init; } = 40_000;
+
+    /// The bandmap keeps the receiver in the middle instead of moving only when
+    /// the receiver nears an edge.
+    public bool BandmapCentred { get; init; }
+
     /// Moves each incoming CW spot by a few tens of hertz, so the operator has
     /// to find the station by ear. N1MM calls it randomising.
     public bool RandomizeSpots { get; init; }