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.

FORK-NOTICE.md

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+# MMD PDF
+
+MMD Steel Group's corporate PDF editor for Windows.
+
+This is a fork of **KillerPDF 1.7.5** by Steve the Killer (https://github.com/SteveTheKiller/KillerPDF),
+licensed **GPL-3.0**. This fork stays GPL-3.0 and its complete corresponding source is this repository.
+
+## What MMD changed
+
+- **Edited text is really removed.** Upstream lays an opaque cover over the original text run and
+  draws the replacement on top, leaving the original glyphs in the file - text extraction returned
+  the old value as well as the new one. `Services/PdfRedactText.cs` walks the page content stream at
+  save time and blanks every text-showing operator whose origin sits inside a cover, so an edited
+  figure is gone from the file rather than hidden.
+- **One theme.** `Themes/MMD.xaml` is the only theme: C8102E red used sparingly, 23221E ink,
+  F1F1EC platinum, FFFFFF paper, square corners, no grain, no glyph shadows. The other twelve themes,
+  the accent overlays and the theme picker are gone. There is no dark mode.
+- **Corporate chrome.** Product name, icon and word-mark are MMD; the upstream author credit in the
+  status bar and the in-app 'install me' button are removed.