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/TerminalStrip/terminalstrip.cpp

@@ -639,7 +639,7 @@ bool TerminalStrip::isBridgeable(const QVector<QSharedPointer<RealTerminal>> &re
 		// Get the physical terminal and pos
 	auto first_physical_terminal = first_real_terminal->physicalTerminal();
 	QVector<shared_physical_terminal> physical_vector{first_physical_terminal};
-	QVector<int> pos_vector{m_physical_terminals.indexOf(first_physical_terminal)};
+	QVector<int> pos_vector{static_cast<int>(m_physical_terminals.indexOf(first_physical_terminal))};
 
 	auto bridge_ = isBridged(first_real_terminal);
 		//bool to know at the end of this function if at least one terminal is not bridged

sources/TerminalStrip/ui/terminalstripmodel.h

@@ -29,9 +29,15 @@
 #include "modelTerminalData.h"
 
 //Code to use QColor as key for QHash
-inline uint qHash(const QColor &key, uint seed) {
-	return qHash(key.name(), seed);
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+inline size_t qHash(const QColor &key, size_t seed = 0) {
+	return qHash(key.rgba(), seed);
 }
+#else
+inline uint qHash(const QColor &key, uint seed) {
+	return qHash(key.rgba(), seed);
+}
+#endif
 
 //needed to use QPointer<Element> as key of QHash
 inline uint qHash(const QPointer<Element> &key, uint seed) {