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.

Controls/Viewer/PdfViewer.Crop.cs

@@ -16,10 +16,10 @@ using Microsoft.Win32;
 using PdfSharpCore.Drawing;
 using PdfSharpCore.Pdf;
 using PdfSharpCore.Pdf.IO;
-using KillerPDF.Services;
+using MmdPdf.Services;
 using PdfPigDoc = UglyToad.PdfPig.PdfDocument;
 
-namespace KillerPDF.Controls
+namespace MmdPdf.Controls
 {
     // Moved from Shell/Crop.cs; the namespace and class line are the only changes. Window members
     // spelled bare here resolve through PdfViewer.Bridge.cs.