About dialog: fix unreadable dark-mode text and missing version

The subtitle and version line used palette(mid), a role meant for 3D
bevel/shadow decoration, not text -- it has poor contrast against the
window background in dark themes. Replaced with a color blended from
the widget's actual text/window palette colors, so it stays readable
(de-emphasized but never low-contrast) in either theme.

Also: QCoreApplication::applicationVersion() was never being set
anywhere, so the dialog always showed "Development build" regardless
of the actual built version. CMake's PROJECT_VERSION is now exposed
to the app via a compile definition and wired into
setApplicationVersion() at startup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main.cpp

@@ -10,6 +10,9 @@ int main(int argc, char* argv[])
     QApplication app(argc, argv);
     app.setOrganizationName(QStringLiteral("FireBugIT"));
     app.setApplicationName(QStringLiteral("OrbitHub"));
+#ifdef ORBITHUB_VERSION_STRING
+    app.setApplicationVersion(QStringLiteral(ORBITHUB_VERSION_STRING));
+#endif
     app.setWindowIcon(createOrbitHubAppIcon());
 
     SessionWindow window;