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.

Features/Viewer/IViewerHost.cs

@@ -1,9 +1,9 @@
 using System.Windows;
 using System.Windows.Input;
 using System.Windows.Controls;
-using KillerPDF.Controls;
+using MmdPdf.Controls;
 
-namespace KillerPDF.Features
+namespace MmdPdf.Features
 {
     /// <summary>
     /// What a document viewer needs from the window around it.