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/ShortcutsOverlay.cs

@@ -3,7 +3,7 @@ using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Media;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     // ============================================================
     // Keyboard-shortcuts overlay: the LIST view.