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.

Services/PdfRasterize.cs

@@ -5,11 +5,11 @@ using Docnet.Core.Models;
 using PdfSharpCore.Drawing;
 using PdfSharpCore.Pdf;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     // ============================================================
     // Document rasterization cores - pure functions over a rendered
-    // source file, no window state (KillerUI refactor; split out of
+    // source file, no window state (MmdPdfUI refactor; split out of
     // FileOperations.cs' Save Flattened and Export Images flows).
     // The shell keeps the dialogs, the annotation burn, and the
     // progress overlay; progress arrives via callback, cancel via