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/FullScreen.cs

@@ -7,7 +7,7 @@ using System.Windows.Media;
 using System.Windows.Media.Animation;
 using System.Windows.Threading;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     public partial class MainWindow
     {
@@ -131,7 +131,7 @@ namespace KillerPDF
 
                 var b = CurrentMonitorBoundsDip();
                 Topmost = true;
-                // #215: Topmost exists only to cover the always-on-top taskbar while KillerPDF is
+                // #215: Topmost exists only to cover the always-on-top taskbar while MmdPdf is
                 // the ACTIVE window. Held unconditionally, it sat over every other program the user
                 // switched to. Yield it on deactivate, take it back on return - browser behavior.
                 Deactivated += FsYieldTopmost;