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/OcrLanguages.cs

@@ -2,12 +2,12 @@ using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     // ============================================================
     // OCR languages - the catalog, install checks and traineddata
     // downloads. Pure helpers over files, settings and HTTP; no
-    // window state. Split out of Ocr.cs (KillerUI refactor).
+    // window state. Split out of Ocr.cs (MmdPdfUI refactor).
     // ============================================================
     internal static class OcrLanguages
     {
@@ -58,7 +58,7 @@ namespace KillerPDF.Services
             // Timeout covers connect + headers; the body is bounded by the cancellation token instead.
             System.Net.ServicePointManager.SecurityProtocol |= System.Net.SecurityProtocolType.Tls12;
             var http = new System.Net.Http.HttpClient { Timeout = TimeSpan.FromSeconds(100) };
-            http.DefaultRequestHeaders.UserAgent.ParseAdd("KillerPDF-OCR");
+            http.DefaultRequestHeaders.UserAgent.ParseAdd("MmdPdf-OCR");
             return http;
         }