feat: rebrand to MMD PDF, single corporate theme, and remove original text under an edit

- Services/PdfRedactText.cs: strip text whose origin falls inside a CoverAnnotation
  from the page content stream at save time, hooked into PdfBurn.DrawAnnotationsIntoDoc.
  Fixes edited values staying recoverable by text extraction.
- Themes/MMD.xaml replaces all thirteen themes; picker and accent strip removed; no dark mode.
- Rename KillerPDF -> MMD PDF across code, resources, packaging and locale strings; new icon.
- Remove the upstream author credit and the in-app install button.

MmdPdf.Tests/PerspectiveWarpTests.cs

@@ -0,0 +1,38 @@
+using System.Windows;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using MmdPdf.Services;
+using Xunit;
+
+namespace MmdPdf.Tests;
+
+public sealed class PerspectiveWarpTests
+{
+    [Fact]
+    public void IdentityCornersPreserveDimensionsAndCornerPixels()
+    {
+        byte[] pixels =
+        {
+            1,2,3,255,  4,5,6,255,
+            7,8,9,255,  10,11,12,255,
+        };
+        var source = BitmapSource.Create(2, 2, 96, 96, PixelFormats.Bgra32, null, pixels, 8);
+        Point[] corners = { new(0,0), new(1,0), new(1,1), new(0,1) };
+
+        var result = PerspectiveWarp.Apply(source, corners);
+        byte[] actual = new byte[16];
+        result.CopyPixels(actual, 8, 0);
+
+        Assert.Equal(2, result.PixelWidth);
+        Assert.Equal(2, result.PixelHeight);
+        Assert.Equal(pixels, actual);
+    }
+
+    [Fact]
+    public void RejectsCollapsedQuadrilateral()
+    {
+        var source = BitmapSource.Create(2, 2, 96, 96, PixelFormats.Bgra32, null, new byte[16], 8);
+        Point[] corners = { new(0,0), new(0,0), new(0,0), new(0,0) };
+        Assert.Throws<System.InvalidOperationException>(() => PerspectiveWarp.Apply(source, corners));
+    }
+}