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/TextFormatterTests.cs

@@ -0,0 +1,69 @@
+using PdfSharpCore.Drawing;
+using PdfSharpCore.Drawing.Layout;
+using PdfSharpCore.Pdf;
+using Xunit;
+
+namespace MmdPdf.Tests
+{
+    /// <summary>
+    /// Regression cover for burning multi-line text annotations into a document (#142).
+    /// CreateBlocks emits a Block(BlockType.LineBreak) per newline, and that constructor never sets
+    /// Text. CreateLayout then skips assigning those blocks a Location, so they keep XPoint(0,0) and
+    /// GetLines' GroupBy lumps them in with the first line. The Justify branch of DrawString called
+    /// block.Text.Trim() on them and threw NullReferenceException.
+    /// Only the Justify path was affected - the other alignments join blocks with string.Join, which
+    /// tolerates nulls. The short DrawString overload defaults to Justify, so MmdPdf hit it.
+    /// </summary>
+    public class TextFormatterTests
+    {
+        static void Draw(string content, TextFormatAlignment? alignment = null)
+        {
+            var doc  = new PdfDocument();
+            var page = doc.AddPage();
+            var gfx  = XGraphics.FromPdfPage(page);
+            var font = new XFont("Segoe UI", 12, XFontStyle.Regular);
+            var rect = new XRect(20, 20, 300, 200);
+            var tf   = new XTextFormatter(gfx);
+
+            if (alignment == null)
+                tf.DrawString(content, font, XBrushes.Black, rect);
+            else
+                tf.DrawString(content, font, XBrushes.Black, rect, alignment);
+        }
+
+        [Fact]
+        public void DrawString_SingleLine_DoesNotThrow()
+        {
+            Draw("Hello world");
+        }
+
+        [Fact]
+        public void DrawString_MultiLine_DoesNotThrow()
+        {
+            // The short overload defaults to Justify - this is the exact call MmdPdf makes when
+            // burning a text annotation, and the one that crashed.
+            Draw("Hello\nworld");
+        }
+
+        [Fact]
+        public void DrawString_MultiLineCrLf_DoesNotThrow()
+        {
+            Draw("Hello\r\nworld");
+        }
+
+        [Fact]
+        public void DrawString_MultiLineLeftAligned_DoesNotThrow()
+        {
+            // What the annotation burn now passes explicitly, matching the on-screen TextBlock.
+            Draw("Hello\nworld", new TextFormatAlignment { Horizontal = XParagraphAlignment.Left });
+        }
+
+        [Fact]
+        public void DrawString_ConsecutiveNewlines_DoesNotThrow()
+        {
+            // A blank line produces two adjacent LineBreak blocks, so the line group is entirely
+            // non-drawable. Guards the empty-after-filter path.
+            Draw("Hello\n\n\nworld");
+        }
+    }
+}