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/PdfScrub.cs

@@ -1,11 +1,11 @@
 using PdfSharpCore.Pdf;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     // ============================================================
     // Pre-save document scrubs - pure functions over a PdfDocument,
     // no window state. Split out of FileOperations.cs and Links.cs
-    // (KillerUI refactor); shared by the GUI save paths, TempReload,
+    // (MmdPdfUI refactor); shared by the GUI save paths, TempReload,
     // the CLI runner and the batch runner.
     // ============================================================
     internal static class PdfScrub
@@ -154,7 +154,7 @@ namespace KillerPDF.Services
             return false;
         }
 
-        // A KillerPDF save fully REWRITES the file, which mathematically invalidates any existing
+        // A MmdPdf save fully REWRITES the file, which mathematically invalidates any existing
         // digital signature: its /ByteRange and digest describe the old bytes (ISO 19005-2, 6.4.3
         // requires the digest to cover the entire file). Carrying the dead signature forward
         // misleads viewers and fails PDF/A validation, so strip signature VALUES (/V) from