Screensavers (#462)

Adds screensavers in addition to the backlight idle off. 
More info in screensavers.md

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

* **New Features**
  * Added a screensaver system with multiple styles (Bouncing Balls, Mystify, Matrix Rain) and a common screensaver interface; auto-starts after inactivity, dismisses on interaction, and supports auto-off/backlight behavior.
  * New Display setting and UI dropdown to choose and persist the screensaver.

* **Documentation**
  * Added comprehensive screensaver docs covering usage, extension, and configuration.

* **Chores**
  * Registered the display-idle service.

* **Bug Fixes**
  * Updated LVGL API calls to match renamed functions.

<sub>✏️ Tip: You can customize this high-level summary in your review settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Tactility/Include/Tactility/settings/DisplaySettings.h

@@ -12,12 +12,21 @@ enum class Orientation {
     PortraitFlipped,
 };
 
+enum class ScreensaverType {
+    None,           // Just black screen
+    BouncingBalls,
+    Mystify,
+    MatrixRain,
+    Count           // Sentinel for bounds checking - must be last
+};
+
 struct DisplaySettings {
     Orientation orientation;
     uint8_t gammaCurve;
     uint8_t backlightDuty;
     bool backlightTimeoutEnabled;
     uint32_t backlightTimeoutMs; // 0 = Never
+    ScreensaverType screensaverType = ScreensaverType::BouncingBalls;
 };
 
 /** Compares default settings with the function parameter to return the difference */