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.

Features/About/IAboutHost.cs

@@ -1,4 +1,4 @@
-namespace KillerPDF.Features
+namespace MmdPdf.Features
 {
     /// <summary>
     /// What AboutController needs from the window hosting it, beyond the shared shell services.
@@ -6,7 +6,7 @@ namespace KillerPDF.Features
     /// Every member is a value or a plain string, never a control, so the controller holds no
     /// reference to a TextBlock or a Button and can be driven by a stub in a test.
     ///
-    /// KillerPDF differs from Killendar's version in one way worth knowing: several of the About
+    /// MmdPdf differs from Killendar's version in one way worth knowing: several of the About
     /// card's lines are built as INLINES rather than set as text - the wordmark is two differently
     /// styled runs, and the tagline, version and alias each carry a hyperlink. Constructing those
     /// is UI work, so it stays in the shell and the controller hands over only the strings and the