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.

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<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Devices/lilygo-tdeck/Source/Trackball/Trackball.cpp

@@ -107,7 +107,7 @@ static void read_cb(lv_indev_t* indev, lv_indev_data_t* data) {
         data->enc_diff = static_cast<int16_t>(clamp(diff, INT16_MIN, INT16_MAX));
 
         if (diff != 0) {
-            lv_disp_trig_activity(nullptr);
+            lv_display_trigger_activity(nullptr);
         }
     } else {
         // Pointer mode: read and clamp cursor position
@@ -127,7 +127,7 @@ static void read_cb(lv_indev_t* indev, lv_indev_data_t* data) {
     data->state = pressed ? LV_INDEV_STATE_PRESSED : LV_INDEV_STATE_RELEASED;
 
     if (pressed) {
-        lv_disp_trig_activity(nullptr);
+        lv_display_trigger_activity(nullptr);
     }
 }