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/UndoTypes.cs

@@ -1,14 +1,14 @@
 using System.Collections.Generic;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     // The undo stack's entry type. Each entry is either an annotation removal or a full document
     // snapshot; AnnotationGroup removes a specific set in one step (a text edit = cover + text).
     //
     // TOP-LEVEL, not nested in MainWindow. The code that pushes undo entries - Annotations.cs and
-    // TextEditing.cs - lives in KillerPDF.Controls, where a type nested in MainWindow only spells
+    // TextEditing.cs - lives in MmdPdf.Controls, where a type nested in MainWindow only spells
     // as MainWindow.UndoEntry; that would mean qualifying roughly 30 call sites for no gain. As
-    // top-level types in KillerPDF they resolve unqualified from the child namespace too.
+    // top-level types in MmdPdf they resolve unqualified from the child namespace too.
     //
     // This also retires the CS0052 chain that made them internal in the first place: DocumentSession
     // had to be internal for the render cache, its UndoStack field is Stack<UndoEntry>, and a field