feat: fit-to-plate splits use full plate work area with preview line

FitToPlate now places split lines at usable-width intervals so each
piece (except the last) fills the entire plate work area. Also adds a
live yellow preview line that follows the cursor during manual split
line placement, and piece dimension labels in the preview regions.

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

OpenNest.Tests/Splitting/SplitLineTests.cs

@@ -42,7 +42,7 @@ public class AutoSplitCalculatorTests
 
         Assert.Single(lines);
         Assert.Equal(CutOffAxis.Vertical, lines[0].Axis);
-        Assert.Equal(50.0, lines[0].Position, 1);
+        Assert.Equal(58.0, lines[0].Position, 1);
     }
 
     [Fact]