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.

Models/ViewTypes.cs

@@ -1,11 +1,11 @@
-namespace KillerPDF
+namespace MmdPdf
 {
     // How a document view lays its pages out, and how it fits them to the viewport.
     //
-    // TOP-LEVEL, not nested in MainWindow. The viewer is a UserControl in KillerPDF.Controls, and
+    // TOP-LEVEL, not nested in MainWindow. The viewer is a UserControl in MmdPdf.Controls, and
     // from there a type nested in MainWindow only spells as MainWindow.ViewMode - which would mean
-    // qualifying 91 references for no gain. As top-level types in KillerPDF they resolve
-    // unqualified from KillerPDF.Controls too (a namespace declaration puts its parent namespaces
+    // qualifying 91 references for no gain. As top-level types in MmdPdf they resolve
+    // unqualified from MmdPdf.Controls too (a namespace declaration puts its parent namespaces
     // in scope), so every call site compiles untouched.
     //
     // internal, not public: nothing outside the assembly has any business with either.