feat(display): night-dim backlight window for screensaver

- DisplaySettings: nightDimEnabled/startHour/endHour/duty
  (default 22:00-07:00 at duty 20), persisted in display.properties
- Display settings page: Night dim switch + Dim from/until hour
  dropdowns + Dim brightness slider
- DisplayIdle: screensaver runs at the dimmed level when the local
  hour is inside the window (wrap-around aware); falls back to the
  regular duty when time is unknown (year < 2025)

Tactility/Source/service/displayidle/DisplayIdle.cpp

@@ -61,6 +61,34 @@ static bool hasDisplayWithBacklight() {
     return result;
 }
 
+/**
+ * Effective backlight duty for the screensaver: the dimmed night level when
+ * night dim is enabled and the local hour is inside the configured window,
+ * otherwise the regular duty. A start == end window covers the whole day.
+ */
+static uint8_t screensaverBacklightDuty(const settings::display::DisplaySettings& displaySettings) {
+    if (!displaySettings.nightDimEnabled) {
+        return displaySettings.backlightDuty;
+    }
+    const std::time_t now = std::time(nullptr);
+    std::tm local_time {};
+    if (localtime_r(&now, &local_time) == nullptr || local_time.tm_year < 125) {
+        return displaySettings.backlightDuty;
+    }
+    const uint8_t hour = static_cast<uint8_t>(local_time.tm_hour);
+    const uint8_t start = displaySettings.nightDimStartHour;
+    const uint8_t end = displaySettings.nightDimEndHour;
+    bool in_window;
+    if (start == end) {
+        in_window = true;
+    } else if (start < end) {
+        in_window = hour >= start && hour < end;
+    } else {
+        in_window = hour >= start || hour < end;
+    }
+    return in_window ? displaySettings.nightDimDuty : displaySettings.backlightDuty;
+}
+
 void DisplayIdleService::stopScreensaver() {
     if (!lvgl_try_lock(100)) {
         // Lock failed - keep flag set to retry on next tick
@@ -142,6 +170,8 @@ void DisplayIdleService::activateScreensaver() {
     if (screensaver) {
         screensaver->start(screensaverOverlay, screenW, screenH);
     }
+
+    setBacklightBrightness(screensaverBacklightDuty(cachedDisplaySettings));
 }
 
 void DisplayIdleService::updateScreensaver() {
@@ -301,7 +331,7 @@ void DisplayIdleService::startMcpScreensaver() {
     screensaverActiveCounter = 0;
     backlightOff = false;
     setBacklightBrightness(cachedDisplaySettings.backlightDuty == 0
-        ? 255 : cachedDisplaySettings.backlightDuty);
+        ? 255 : screensaverBacklightDuty(cachedDisplaySettings));
 
     lv_coord_t screenW = lv_display_get_horizontal_resolution(nullptr);
     lv_coord_t screenH = lv_display_get_vertical_resolution(nullptr);