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.

MmdPdf.Tests/ProtocolRegistrarTests.cs

@@ -0,0 +1,23 @@
+using MmdPdf.Services;
+using Xunit;
+
+namespace MmdPdf.Tests;
+
+public sealed class ProtocolRegistrarTests
+{
+    [Fact]
+    public void ParsesEncodedHttpsPdfUrl()
+    {
+        Assert.True(ProtocolRegistrar.TryGetTargetUrl(
+            "mmdpdf://open?url=https%3A%2F%2Fexample.com%2Ffile.pdf%3Fx%3D1", out var target));
+        Assert.Equal("https://example.com/file.pdf?x=1", target!.AbsoluteUri);
+    }
+
+    [Theory]
+    [InlineData("mmdpdf://open?url=http%3A%2F%2Fexample.com%2Ffile.pdf")]
+    [InlineData("mmdpdf://open?url=file%3A%2F%2Fc%3A%2Fsecret.pdf")]
+    [InlineData("mmdpdf://wrong?url=https%3A%2F%2Fexample.com%2Ffile.pdf")]
+    [InlineData("https://example.com/file.pdf")]
+    public void RejectsUnsafeOrUnrelatedLaunches(string value)
+        => Assert.False(ProtocolRegistrar.TryGetTargetUrl(value, out _));
+}