Add a 60 kHz bandmap zoom, a mode-wide zoom and denser scale ticks

The zoom list gains 60 kHz and a "mode" level that shows the CW, digital
or phone part of the band the receiver is in, before "band". For views
from 40 to 200 kHz wide, the scale picks the smallest round step that
keeps three lines of text between ticks. The font size buttons are
removed; Ctrl with the wheel or plus and minus still change the size.

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

tests/Nonemm.Spotting.Tests/BandmapSliceTests.cs

@@ -61,4 +61,34 @@ public class BandmapSliceTests
 
         Assert.Equal(Khz(14_005), low);
     }
+
+    private static readonly BandSegments Segments = new(
+        Bands.Band20M, CwHigh: Khz(14_150), DigitalLow: Khz(14_070), DigitalHigh: Khz(14_099));
+
+    [Theory]
+    [InlineData(14_025, 14_000, 14_150)]
+    [InlineData(14_080, 14_070, 14_099)]
+    [InlineData(14_250, 14_150, 14_350)]
+    public void ThePartOfTheBandIsTheOneTheReceiverIsIn(double receiver, double low, double high)
+    {
+        (Frequency from, Frequency to) = BandmapSlice.Part(Bands.Band20M, Segments, Khz(receiver));
+
+        Assert.Equal((Khz(low), Khz(high)), (from, to));
+    }
+
+    [Fact]
+    public void ABandWithoutBoundariesShowsTheWholeBandAsAPart()
+    {
+        (Frequency from, Frequency to) = BandmapSlice.Part(Bands.Band20M, null, Khz(14_025));
+
+        Assert.Equal((Bands.Band20M.Low, Bands.Band20M.High), (from, to));
+    }
+
+    [Fact]
+    public void ThePartIsStretchedToIncludeAReceiverOutsideTheBand()
+    {
+        (_, Frequency to) = BandmapSlice.Part(Bands.Band20M, Segments, Khz(14_360));
+
+        Assert.Equal(Khz(14_360), to);
+    }
 }