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.

Shell/ExternalOpen.cs

@@ -2,9 +2,9 @@ using System.IO;
 using System.Net.Http;
 using System.Threading.Tasks;
 using System.Windows;
-using KillerPDF.Services;
+using MmdPdf.Services;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     /// <summary>
     /// Single-instance entry points: App forwards a second launch's file path here rather than
@@ -88,8 +88,8 @@ namespace KillerPDF
             {
                 try { File.Delete(temp); } catch { }
                 KillerDialog.Show(this,
-                    $"KillerPDF could not open the browser PDF.\n\n{ex.Message}",
-                    "KillerPDF", MessageBoxButton.OK, MessageBoxImage.Warning);
+                    $"MmdPdf could not open the browser PDF.\n\n{ex.Message}",
+                    "MmdPdf", MessageBoxButton.OK, MessageBoxImage.Warning);
             }
         }