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.

build/payload-files.txt

@@ -2,8 +2,8 @@ CommunityToolkit.Mvvm.dll
 de/PdfSharpCore.resources.dll
 Docnet.Core.dll
 ICSharpCode.SharpZipLib.dll
-KillerPDF.App.exe
-KillerPDF.App.exe.config
+MmdPdf.App.exe
+MmdPdf.App.exe.config
 LICENSE
 Microsoft.Bcl.AsyncInterfaces.dll
 Microsoft.Bcl.HashCode.dll