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.

Controls/Viewer/PdfViewer.TabStrip.cs

@@ -5,7 +5,7 @@ using System.Windows.Controls;
 using System.Windows.Input;
 using System.Windows.Media;
 
-namespace KillerPDF.Controls
+namespace MmdPdf.Controls
 {
     // This pane's tab band: which tabs are in the strip, which of them owns an edge, what the card's
     // top corners do, where the focus ring runs, and the drag that reorders them or hands one to the
@@ -413,7 +413,7 @@ namespace KillerPDF.Controls
             bool firstInactiveRetro = retro && _sessions.Any(t => t.IsStripVisible && t.IsFirst && !t.IsActive);
             bool lastInactiveRetro = retro && _sessions.Any(t => t.RetroLastInactive);
             // The XAML declares these with NO Background - unlike KillerShell's copy, which paints
-            // them PrimaryBrush directly in markup, KillerPDF's accent key is only known at runtime
+            // them PrimaryBrush directly in markup, MmdPdf's accent key is only known at runtime
             // (SelectionAccent, resolved the same way SetFocusHalo resolves the card border). Without
             // this they toggle Visible and still draw nothing: a transparent Border is invisible
             // whatever its Visibility says.