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/Include/Tactility/settings/DisplaySettings.h

@@ -37,6 +37,14 @@ struct DisplaySettings {
     bool backlightTimeoutEnabled;
     uint32_t backlightTimeoutMs; // 0 = Never
     ScreensaverType screensaverType = ScreensaverType::BouncingBalls;
+    // Night dim: when enabled and the local hour falls inside
+    // [nightDimStartHour, nightDimEndHour), the screensaver runs at
+    // nightDimDuty instead of backlightDuty. A start == end window
+    // means the whole day.
+    bool nightDimEnabled = false;
+    uint8_t nightDimStartHour = 22;
+    uint8_t nightDimEndHour = 7;
+    uint8_t nightDimDuty = 20;
 };
 
 /** Compares default settings with the function parameter to return the difference */