feat: 'Prefer reduced motion' setting (accessibility / animation opt-out)

Config::reducedMotion (default false = playful animations ON). Surfaced as a
checkbox on first run (wizard PageMenu) and in Settings → Appearance —
unchecked by default, matching the OS 'Reduce Motion' convention. The mining
tab's AnimatedImages now freeze when it's set (MiningBackend exposes it to
mining.qml). Foundation for gating the ambient silliness to come.

src/SettingsDialog.cpp

@@ -139,6 +139,12 @@ void Settings::setupAppearanceTab() {
        emit updateBalance();
     });
 
+    // [Prefer reduced motion] wowlet: gate playful animations (mining tab, loaders, ...)
+    ui->checkBox_reducedMotion->setChecked(conf()->get(Config::reducedMotion).toBool());
+    connect(ui->checkBox_reducedMotion, &QCheckBox::toggled, [this](bool toggled){
+       conf()->set(Config::reducedMotion, toggled);
+    });
+
     // [Preferred fiat currency]
     QStringList availableFiatCurrencies = appData()->prices.rates.keys();
     for (const auto &currency : availableFiatCurrencies) {