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/FlyoutPlacement.cs

@@ -2,11 +2,11 @@ using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Controls.Primitives;
 
-namespace KillerPDF
+namespace MmdPdf
 {
     /// <summary>
     /// Where every flyout opens: the bottom corner of the content pane beside the rail.
-    /// (From KillerUI/Shell/FlyoutPlacement.cs - the family flyout standard.)
+    /// (From MmdPdfUI/Shell/FlyoutPlacement.cs - the family flyout standard.)
     ///
     /// That rail-adjacent corner is the answer because of what bounds it, and all three matter:
     ///   - it is INSIDE the window, so a flyout never hangs over the desktop;