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.

Models/PageThumbnailVm.cs

@@ -8,7 +8,7 @@ using System.Windows.Media.Imaging;
 using Docnet.Core;
 using Docnet.Core.Models;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     /// <summary>
     /// ViewModel for a single page thumbnail in the sidebar PageList.
@@ -88,7 +88,7 @@ namespace KillerPDF
                 using var pr = docReader.GetPageReader(pageIndex);
                 int tw  = pr.GetPageWidth();
                 int th  = pr.GetPageHeight();
-                var raw = KillerPDF.Services.PdfiumInterop.RenderPageWithAnnotations(filePath, pageIndex, tw, th)
+                var raw = MmdPdf.Services.PdfiumInterop.RenderPageWithAnnotations(filePath, pageIndex, tw, th)
                     ?? pr.GetImage();   // #141
                 if (tw <= 0 || th <= 0 || raw == null || raw.Length < tw * th * 4)
                     return null;