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/ExportImagesDialog.cs

@@ -1,7 +1,7 @@
 using System.Windows;
 using System.Windows.Controls;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     // Export pages as images (#132): PNG/JPEG + DPI + page range, themed via DialogChrome like
     // Document Info. The destination and base file name are picked afterwards with the standard
@@ -21,7 +21,7 @@ namespace KillerPDF
         /// (#207) opens the same dialog scoped to the clicked page(s), still editable.</summary>
         public ExportImagesDialog(Window owner, string presetRange = "")
         {
-            Title = "KillerPDF - " + L("Str_ExportImg_Suffix");
+            Title = "MmdPdf - " + L("Str_ExportImg_Suffix");
             // Width follows the caption. "Export Pages as Images" is 22 characters in en-US and
             // up to 35 translated, which ran the title under the close button at a fixed 380 (#223).
             MinWidth = 380;
@@ -68,7 +68,7 @@ namespace KillerPDF
             row.Margin = new Thickness(0, 8, 0, 0);
             body.Children.Add(row);
 
-            Content = DialogChrome.Frame(this, Owner, "KillerPDF - " + L("Str_ExportImg_Suffix"),
+            Content = DialogChrome.Frame(this, Owner, "MmdPdf - " + L("Str_ExportImg_Suffix"),
                 () => { Confirmed = false; Close(); }, body);
 
             Loaded += (_, _2) => _dpi.Focus();