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/PdfImport.cs

@@ -8,12 +8,12 @@ using PdfSharpCore.Drawing;
 using PdfSharpCore.Pdf;
 using PdfSharpCore.Pdf.IO;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     // ============================================================
     // File import/repair helpers - pure functions over paths and
     // PdfDocuments, no window state. Split out of FileOperations.cs
-    // and ImportAndZip.cs (KillerUI refactor); shared by the GUI
+    // and ImportAndZip.cs (MmdPdfUI refactor); shared by the GUI
     // open/merge/repair paths, TempReload, and the CLI. The one
     // import helper NOT here is TryPdfiumStripEncryption - it rides
     // the shared PDFium interop block (and its lock), which stays