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/SearchServiceTests.cs

@@ -0,0 +1,52 @@
+using MmdPdf.Services;
+using Xunit;
+
+namespace MmdPdf.Tests
+{
+    public class SearchServiceTests
+    {
+        private readonly SearchService _svc = new();
+
+        [Fact]
+        public void Search_EmptyQuery_ReturnsEmpty()
+        {
+            var result = _svc.Search("irrelevant.pdf", "");
+            Assert.Empty(result.ResultPages);
+            Assert.Equal(0, result.TotalHits);
+        }
+
+        [Fact]
+        public void Search_WhitespaceQuery_ReturnsEmpty()
+        {
+            var result = _svc.Search("irrelevant.pdf", "   ");
+            Assert.Empty(result.ResultPages);
+            Assert.Equal(0, result.TotalHits);
+        }
+
+        [Fact]
+        public void Search_EmptyFilePath_ReturnsEmpty()
+        {
+            var result = _svc.Search("", "hello");
+            Assert.Empty(result.ResultPages);
+            Assert.Equal(0, result.TotalHits);
+        }
+
+        [Fact]
+        public void Search_MissingFile_ReturnsEmpty()
+        {
+            // Should not throw; non-existent file produces no results.
+            var result = _svc.Search(@"C:\does\not\exist.pdf", "hello");
+            Assert.Empty(result.ResultPages);
+            Assert.Equal(0, result.TotalHits);
+        }
+
+        [Fact]
+        public void SearchResult_PageRects_EmptyByDefault()
+        {
+            var result = new SearchResult();
+            Assert.Empty(result.PageRects);
+            Assert.Empty(result.ResultPages);
+            Assert.Equal(0, result.TotalHits);
+        }
+    }
+}