Fix remaining Qt6 build warnings (narrowing, nodiscard, qHash)

Clears the 9 non-deprecation warnings from the Qt6 build:
- qHash(QColor): hash rgba() (unambiguous QRgb) instead of name(), and
  use the size_t seed signature on Qt6 (guarded for Qt5). Fixes the
  ambiguous-overload warning in terminalstripmodel.h.
- Two qsizetype->int narrowings in brace-init: explicit static_cast<int>
  (elementscene.cpp, terminalstrip.cpp).
- main.cpp: keep the QtConcurrent::run QFuture in a [[maybe_unused]]
  variable (nodiscard).
- qetapp.cpp: guard the stylesheet load on QFile::open() succeeding
  (nodiscard) instead of ignoring the result.

sources/main.cpp

@@ -289,7 +289,7 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 	// here guarantees the singleton is fully built before the worker runs.
 	MachineInfo::instance();
 
-	QtConcurrent::run([=]()
+	[[maybe_unused]] auto startup_future = QtConcurrent::run([=]()
 	{
 		// for debugging
 		qInstallMessageHandler(myMessageOutput);