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/SidebarOutline.cs

@@ -16,10 +16,10 @@ using Microsoft.Win32;
 using PdfSharpCore.Drawing;
 using PdfSharpCore.Pdf;
 using PdfSharpCore.Pdf.IO;
-using KillerPDF.Services;
+using MmdPdf.Services;
 using PdfPigDoc = UglyToad.PdfPig.PdfDocument;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     public partial class MainWindow
     {
@@ -657,7 +657,7 @@ namespace KillerPDF
         }
 
         // CountOutlines, RemoveOutlineRecursive and the stale-link-key scrubs live in
-        // Services/PdfOutlines.cs (KillerUI refactor) - pure functions over the outline tree.
+        // Services/PdfOutlines.cs (MmdPdfUI refactor) - pure functions over the outline tree.
 
         /// <summary>Rebuilds the outline panel after an edit, keeping every branch's expand/collapse
         /// state (the PdfOutline objects survive the rebuild, so they key the state).</summary>