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/StartupTrace.cs

@@ -4,15 +4,15 @@ using System.Globalization;
 using System.IO;
 using System.Text;
 
-namespace KillerPDF.Services
+namespace MmdPdf.Services
 {
     /// <summary>
     /// Opt-in startup timing used by the release benchmark. Normal launches do no file I/O.
-    /// Set KILLERPDF_STARTUP_TRACE to an output path before starting the process.
+    /// Set MMDPDF_STARTUP_TRACE to an output path before starting the process.
     /// </summary>
     internal static class StartupTrace
     {
-        private const string TraceEnvironmentVariable = "KILLERPDF_STARTUP_TRACE";
+        private const string TraceEnvironmentVariable = "MMDPDF_STARTUP_TRACE";
         private static readonly object Gate = new object();
         private static readonly Stopwatch Clock = Stopwatch.StartNew();
         private static readonly string? OutputPath = Environment.GetEnvironmentVariable(TraceEnvironmentVariable);
@@ -36,7 +36,7 @@ namespace KillerPDF.Services
                     {
                         _headerWritten = true;
                         var process = Process.GetCurrentProcess();
-                        sb.Append("# KillerPDF startup trace | pid=")
+                        sb.Append("# MmdPdf startup trace | pid=")
                           .Append(process.Id)
                           .Append(" | processStartUtc=")
                           .Append(process.StartTime.ToUniversalTime().ToString("O", CultureInfo.InvariantCulture))