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/ViewerState.cs

@@ -1,7 +1,7 @@
 using System.Collections.Generic;
 using System.Windows.Controls;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     /// <summary>
     /// Everything ONE document view owns. Split pane needs two of these; each PdfViewer control
@@ -15,7 +15,7 @@ namespace KillerPDF
     /// and zoom; it does NOT need a second copy of the per-document machinery, because each
     /// pane will simply own its own set of sessions.
     ///
-    /// TOP-LEVEL, not nested in MainWindow. The viewer lives in KillerPDF.Controls and cannot own a
+    /// TOP-LEVEL, not nested in MainWindow. The viewer lives in MmdPdf.Controls and cannot own a
     /// type nested in the window without every reference spelling out MainWindow.ViewerState.
     /// ViewMode and FitMode live in Models/ViewTypes.cs for the same reason.
     /// </summary>