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/Signing/PdfSigner.cs

@@ -5,11 +5,11 @@ using PdfSharp.Pdf;
 using PdfSharp.Pdf.IO;
 using PdfSharp.Pdf.Signatures;
 
-namespace KillerPDF.Services.Signing
+namespace MmdPdf.Services.Signing
 {
     /// <summary>
     /// Cryptographic (PAdES / PKCS#7) PDF signing, isolated from the rest of the app. Everything else
-    /// in KillerPDF uses PdfSharpCore; this module uses PDFsharp 6.2 (the <c>PdfSharp.*</c> namespace),
+    /// in MmdPdf uses PdfSharpCore; this module uses PDFsharp 6.2 (the <c>PdfSharp.*</c> namespace),
     /// and the two coexist without clashing. There are deliberately no WPF or Windows-only types here,
     /// so the whole module ports to Avalonia / Linux / Mac unchanged.
     ///