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.

Services/SignatureStore.cs

@@ -1,6 +1,6 @@
 using System.Text.Json;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     internal sealed class SignatureStore
     {
@@ -9,7 +9,7 @@ namespace KillerPDF.Services
 
         private static readonly string DefaultDir  = System.IO.Path.Combine(
             Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData),
-            "KillerPDF");
+            "MmdPdf");
 
         public SignatureStore()
             : this(DefaultDir, System.IO.Path.Combine(DefaultDir, "signatures.json")) { }