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/SignatureStoreTests.cs

@@ -0,0 +1,60 @@
+using MmdPdf.Services;
+using Xunit;
+
+namespace MmdPdf.Tests
+{
+    public class SignatureStoreTests
+    {
+        [Fact]
+        public void NewStore_HasEmptySignatures()
+        {
+            var store = new SignatureStore();
+            Assert.Empty(store.Signatures);
+        }
+
+        [Fact]
+        public void Add_IncreasesCount()
+        {
+            var store = new SignatureStore();
+            store.Add(new SavedSignature { Name = "Test" });
+            Assert.Single(store.Signatures);
+        }
+
+        [Fact]
+        public void Remove_DecreasesCount()
+        {
+            var store = new SignatureStore();
+            var sig = new SavedSignature { Name = "Test" };
+            store.Add(sig);
+            store.Remove(sig);
+            Assert.Empty(store.Signatures);
+        }
+
+        [Fact]
+        public void RoundTrip_PersistAndLoad()
+        {
+            var dir  = System.IO.Path.Combine(System.IO.Path.GetTempPath(), Guid.NewGuid().ToString("N"));
+            System.IO.Directory.CreateDirectory(dir);
+            var file = System.IO.Path.Combine(dir, "sig_test.json");
+
+            try
+            {
+                var store1 = new SignatureStore(dir, file);
+                store1.Add(new SavedSignature { Name = "Alpha", CanvasWidth = 400, CanvasHeight = 150 });
+                store1.Add(new SavedSignature { Name = "Beta",  CanvasWidth = 300, CanvasHeight = 100 });
+                store1.Persist();
+
+                var store2 = new SignatureStore(dir, file);
+                store2.Load();
+
+                Assert.Equal(2, store2.Signatures.Count);
+                Assert.Equal("Alpha", store2.Signatures[0].Name);
+                Assert.Equal("Beta",  store2.Signatures[1].Name);
+            }
+            finally
+            {
+                System.IO.Directory.Delete(dir, recursive: true);
+            }
+        }
+    }
+}