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/devices/TdeckKeyboard.cpp

@@ -52,7 +52,7 @@ static void keyboard_read_callback(lv_indev_t* indev, lv_indev_data_t* data) {
             data->state = LV_INDEV_STATE_PRESSED;
             // TODO: Avoid performance hit by calling loadOrGetDefault() on each key press
             // Ensure LVGL activity is triggered so idle services can wake the display
-            lv_disp_trig_activity(nullptr);
+            lv_display_trigger_activity(nullptr);
 
             // Actively wake display/backlights immediately on key press (independent of idle tick)
             // Restore display backlight if off (we assume duty 0 means dimmed)