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/PdfScrubTests.cs

@@ -0,0 +1,36 @@
+using MmdPdf.Services;
+using PdfSharpCore.Drawing;
+using PdfSharpCore.Pdf;
+using Xunit;
+
+namespace MmdPdf.Tests;
+
+public sealed class PdfScrubTests
+{
+    [Fact]
+    public void ScrubDegenerateCropBoxes_RemovesCropOutsideMediaBox()
+    {
+        using var doc = new PdfDocument();
+        var page = doc.AddPage();
+        page.MediaBox = new PdfRectangle(new XPoint(0, 0), new XPoint(1191, 842));
+        page.CropBox = new PdfRectangle(new XPoint(0, 0), new XPoint(842, 1191));
+        page.Rotate = 90;
+
+        PdfScrub.ScrubDegenerateCropBoxes(doc);
+
+        Assert.Null(page.Elements["/CropBox"]);
+    }
+
+    [Fact]
+    public void ScrubDegenerateCropBoxes_PreservesValidInsetCrop()
+    {
+        using var doc = new PdfDocument();
+        var page = doc.AddPage();
+        page.MediaBox = new PdfRectangle(new XPoint(0, 0), new XPoint(612, 792));
+        page.CropBox = new PdfRectangle(new XPoint(18, 24), new XPoint(594, 768));
+
+        PdfScrub.ScrubDegenerateCropBoxes(doc);
+
+        Assert.NotNull(page.Elements["/CropBox"]);
+    }
+}