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/Search/ISearchHost.cs

@@ -1,4 +1,4 @@
-namespace KillerPDF.Features
+namespace MmdPdf.Features
 {
     /// <summary>
     /// What SearchController needs from the window hosting it, beyond the shared shell services.

Features/Search/SearchController.cs

@@ -1,11 +1,11 @@
-using KillerPDF.Services;
+using MmdPdf.Services;
 
-namespace KillerPDF.Features
+namespace MmdPdf.Features
 {
     /// <summary>
     /// The document-search state machine: runs SearchService over the working file, keeps the
     /// per-page rect map and the flat reading-ordered match list, and steps the match cursor.
-    /// Moved out of Search.cs (MainWindow) in the KillerUI refactor.
+    /// Moved out of Search.cs (MainWindow) in the MmdPdfUI refactor.
     ///
     /// Holds no controls. Talks to the window only through <see cref="ISearchHost"/>; the search
     /// bar UI, debounce, and the highlight painting stay in the shell half (Shell/Search.cs).