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.xaml.cs

@@ -1,9 +1,9 @@
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Input;
-using KillerPDF.Features;
+using MmdPdf.Features;
 
-namespace KillerPDF.Controls
+namespace MmdPdf.Controls
 {
     /// <summary>
     /// One document view: its tab strip, its card and everything inside. Two instances make the
@@ -52,7 +52,7 @@ namespace KillerPDF.Controls
         /// SetResourceReference, not a brush snapshot, on both states: an assigned brush would not
         /// follow a live theme switch.
         ///
-        /// "SelectionAccent", not "AccentBrush". KillerPDF uses the older family resource set
+        /// "SelectionAccent", not "AccentBrush". MmdPdf uses the older family resource set
         /// (BgCanvas / AccentLogo / TextPrimary) and has no AccentBrush key in any theme.
         /// SetResourceReference to a missing key does not throw, it silently leaves the property
         /// unset, which blanks the border instead of accenting it.