Debounce settings writes and save them on SIGTERM

- DebouncedStore, taken out of WindowSettings, now also saves the opacity
  slider once a drag ends instead of on every step
- SIGTERM/SIGINT (how KWin stops the input method) quit normally, so
  pending geometry and settings are written
- The slider's minimum opacity comes from Settings instead of a copy in QML

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/QuitOnSignal.cpp

@@ -0,0 +1,44 @@
+#include "QuitOnSignal.h"
+
+#include <QCoreApplication>
+#include <QSocketNotifier>
+
+#include <csignal>
+#include <sys/signalfd.h>
+#include <unistd.h>
+
+namespace {
+
+sigset_t quitSignals()
+{
+    sigset_t set;
+    sigemptyset(&set);
+    sigaddset(&set, SIGTERM);
+    sigaddset(&set, SIGINT);
+    return set;
+}
+
+} // namespace
+
+void blockQuitSignals()
+{
+    const sigset_t set = quitSignals();
+    pthread_sigmask(SIG_BLOCK, &set, nullptr);
+}
+
+void quitOnSignal()
+{
+    const sigset_t set = quitSignals();
+    const int fd = signalfd(-1, &set, SFD_CLOEXEC | SFD_NONBLOCK);
+    if (fd < 0) {
+        qWarning("signalfd failed: settings may not be saved on SIGTERM");
+        pthread_sigmask(SIG_UNBLOCK, &set, nullptr);
+        return;
+    }
+    auto *notifier = new QSocketNotifier(fd, QSocketNotifier::Read, QCoreApplication::instance());
+    QObject::connect(notifier, &QSocketNotifier::activated, notifier, [fd] {
+        signalfd_siginfo info;
+        while (read(fd, &info, sizeof info) == sizeof info) { }
+        QCoreApplication::quit();
+    });
+}

src/app/QuitOnSignal.h

@@ -0,0 +1,8 @@
+#pragma once
+
+// KWin stops the input method with SIGTERM. Turns SIGTERM and SIGINT into a
+// normal quit, so destructors run and pending settings are written.
+// Call before QGuiApplication: threads Qt starts must inherit the blocked mask.
+void blockQuitSignals();
+// Call once the application exists.
+void quitOnSignal();