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.

Features/Ocr/IOcrHost.cs

@@ -1,7 +1,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-namespace KillerPDF.Features
+namespace MmdPdf.Features
 {
     /// <summary>
     /// What OcrController needs from the window hosting it, beyond the shared shell services.