Fix UI inconsistencies and improve dark mode appearance

This commit addresses multiple UI issues related to theme switching and
dark mode styling:

1. Fix spinbox height inconsistency between light and dark modes
   - Add changeEvent() handler in AmountSpinBox to invalidate cached
     size hint when theme/style/palette changes
   - Force updateGeometry() on all visible widgets after theme toggle
   - Add min-height and padding to dark mode spinbox CSS to match
     light mode dimensions

2. Improve spinbox button visibility in dark mode
   - Add visible background and borders to up/down buttons
   - Use dark grey arrows (#808080) instead of white for better contrast
   - Add hover states with appropriate dark theme colors
   - Set proper button width (16px) for consistent appearance

3. Enhanced theme switching
   - Invalidate widget geometry caches when switching between themes
   - Ensure consistent rendering between native (light) and stylesheet
     (dark) rendering modes

Files modified:
- src/qt/palladiumamountfield.cpp: Add changeEvent() handler
- src/qt/palladiumgui.cpp: Force geometry update after theme change
- src/qt/res/styles/dark.qss: Improve spinbox styling with proper
  dimensions, button visibility, and dark-appropriate colors

This ensures a consistent user experience across both themes with
properly sized widgets and visible, theme-appropriate controls

src/qt/palladiumamountfield.cpp

@@ -190,6 +190,19 @@ protected:
         return QAbstractSpinBox::event(event);
     }
 
+    void changeEvent(QEvent *event)
+    {
+        if (event->type() == QEvent::StyleChange ||
+            event->type() == QEvent::PaletteChange ||
+            event->type() == QEvent::FontChange)
+        {
+            // Invalidate cached size hint when theme/style changes
+            cachedMinimumSizeHint = QSize();
+            updateGeometry();
+        }
+        QAbstractSpinBox::changeEvent(event);
+    }
+
     StepEnabled stepEnabled() const
     {
         if (isReadOnly()) // Disable steps when AmountSpinBox is read-only

src/qt/palladiumgui.cpp

@@ -1544,14 +1544,22 @@ void PalladiumGUI::toggleTheme()
         // 1. Standard Theme (Weiß)
         qApp->setStyleSheet("");
         qApp->setPalette(style()->standardPalette());
-        
+
         // 2. Speichern, dass er aus ist
         settings.setValue("darkModeEnabled", false);
-        
+
         if (modalOverlay) {
             modalOverlay->setStyleSheet("");
         }
     }
+
+    // Force geometry recalculation for all widgets after theme change
+    // This ensures proper sizing when switching between native (light) and stylesheet (dark) rendering
+    for (QWidget *widget : qApp->allWidgets()) {
+        if (widget && widget->isVisible()) {
+            widget->updateGeometry();
+        }
+    }
 }
 
 void PalladiumGUI::checkUpdate()

src/qt/res/styles/dark.qss

@@ -432,6 +432,8 @@ PalladiumAmountField QAbstractSpinBox {
     background-color: #1E1E1E;
     color: #FFFFFF;
     border: 1px solid #4A4A4A;
+    min-height: 26px;
+    padding: 2px;
 }
 
 PalladiumAmountField QAbstractSpinBox:hover {
@@ -444,6 +446,49 @@ PalladiumAmountField QAbstractSpinBox:focus {
     background-color: #252525;
 }
 
+/* Fix for spinbox up/down buttons visibility */
+PalladiumAmountField QAbstractSpinBox::up-button,
+PalladiumAmountField QAbstractSpinBox::down-button {
+    background-color: #2A2A2A;
+    border: 1px solid #3A3A3A;
+    width: 16px;
+}
+
+PalladiumAmountField QAbstractSpinBox::up-button:hover,
+PalladiumAmountField QAbstractSpinBox::down-button:hover {
+    background-color: #3A3A3A;
+    border: 1px solid #4A4A4A;
+}
+
+PalladiumAmountField QAbstractSpinBox::up-button:pressed,
+PalladiumAmountField QAbstractSpinBox::down-button:pressed {
+    background-color: #007BFF;
+}
+
+PalladiumAmountField QAbstractSpinBox::up-arrow {
+    image: none;
+    width: 8px;
+    height: 8px;
+    border-left: 4px solid transparent;
+    border-right: 4px solid transparent;
+    border-bottom: 6px solid #808080;
+}
+
+PalladiumAmountField QAbstractSpinBox::down-arrow {
+    image: none;
+    width: 8px;
+    height: 8px;
+    border-left: 4px solid transparent;
+    border-right: 4px solid transparent;
+    border-top: 6px solid #808080;
+}
+
+PalladiumAmountField QAbstractSpinBox::up-arrow:hover,
+PalladiumAmountField QAbstractSpinBox::down-arrow:hover {
+    border-bottom-color: #A0A0A0;
+    border-top-color: #A0A0A0;
+}
+
 PalladiumAmountField QComboBox {
     background-color: #1E1E1E;
     color: #E0E0E0;