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.

Controls/Viewer/PdfViewer.TextEditing.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.Controls
+namespace MmdPdf.Controls
 {
     // Moved from Shell/TextEditing.cs; the namespace and class line are the only changes. Window
     // members spelled bare here resolve through PdfViewer.Bridge.cs.
@@ -748,7 +748,7 @@ namespace KillerPDF.Controls
                 string family = Services.FontCoverage.PickFamily(want, ta.Content);
                 string missing = Services.FontCoverage.UncoveredChars(family, ta.Content);
                 if (missing.Length == 0) return;
-                KillerDialog.Show(Host!.Window, string.Format(Loc("Str_Font_NoGlyphs"), missing), "KillerPDF",
+                KillerDialog.Show(Host!.Window, string.Format(Loc("Str_Font_NoGlyphs"), missing), "MmdPdf",
                     MessageBoxButton.OK, MessageBoxImage.Warning);
             }
             catch { /* the warning must never be the thing that breaks placing text */ }