pdf: text that names no colour comes out black, not the palette

A QTextDocument takes its default text colour from the application
palette, and markdown names no colour at all: in a window running a dark
palette -- which is what a phone in dark mode hands Qt -- every plain
paragraph was laid out #eeeeee on a white page. Printed, that is a blank
document; on screen it reads as "the font is oddly faint". The first
PDFs off the phone were exactly that.

- PdfTools: paperDocument() sets the default stylesheet (body colour
  #000000) before setHtml, and writePdf() lays that out. A colour the
  document does name still wins, so an .html handed to the tool keeps its
  own -- checked with a #cc0000 document under both palettes.
- --uitest: draws that document on a white image under a dark palette and
  looks for ink (238 before the fix, 0 after).

Verified on the NX789J: md_to_pdf on the device now writes CID TrueType
NotoSansCJKHK-Regular/-Bold text whose page renders at min=0.

app/src/ui/UiTest.cpp

@@ -13,6 +13,7 @@
 #include <QBuffer>
 #include <QFile>
 #include <QImage>
+#include <QPalette>
 #include <QJsonDocument>
 #include <QJsonObject>
 #include <QPainter>
@@ -22,14 +23,17 @@
 #include <QQmlContext>
 #include <QQuickWindow>
 #include <QTest>
+#include <QTextDocument>
 
 #include <QVariantMap>
 
 #include <cstdio>
 #include <cstring>
+#include <memory>
 
 #include "cst_agent.h"
 #include "cst_attach.h"
+#include "core/ToolProvider.h"
 #include "ui/SessionImages.h"
 #include "ui/ThemeIcons.h"
 
@@ -482,6 +486,36 @@ int runUiTest(AgentCore &core, const QString &size) {
     }
   }
 
+  /* Paper is white whatever the window's palette says: markdown carries no
+   * colour, so a dark window turned every plain paragraph #eeeeee -- a PDF
+   * that looked blank. Draw it on a white image and look for ink. */
+  {
+    const QPalette was = QGuiApplication::palette();
+    QPalette darkPalette = was;
+    darkPalette.setColor(QPalette::Text, QColor(0xee, 0xee, 0xee));
+    darkPalette.setColor(QPalette::WindowText, QColor(0xee, 0xee, 0xee));
+    QGuiApplication::setPalette(darkPalette);
+    std::unique_ptr<QTextDocument> page(app::paperDocument(
+        QStringLiteral("<html><body>text that names no colour of its own"
+                       "</body></html>")));
+    QImage paper(320, 60, QImage::Format_RGB32);
+    paper.fill(0xffffffffu);
+    QPainter ink(&paper);
+    page->drawContents(&ink);
+    ink.end();
+    int darkest = 255;
+    for (int y = 0; y < paper.height(); y++) {
+      const QRgb *row = reinterpret_cast<const QRgb *>(paper.constScanLine(y));
+      for (int x = 0; x < paper.width(); x++)
+        darkest = qMin(darkest, qGray(row[x]));
+    }
+    check(darkest < 64,
+          QStringLiteral("a dark window still lays out black ink on paper "
+                         "(darkest pixel %1)")
+              .arg(darkest));
+    QGuiApplication::setPalette(was);
+  }
+
   /* new sessions: the engine moves, the view empties, the store grows */
   const int before = core.sessions().size();
   clickMouse(win, QStringLiteral("newSessionButton"));