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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Tactility/Source/service/keyboardidle/KeyboardIdle.cpp

@@ -32,7 +32,7 @@ class KeyboardIdleService final : public Service {
         // Query LVGL inactivity once for both checks
         uint32_t inactive_ms = 0;
         if (lvgl::lock(100)) {
-            inactive_ms = lv_disp_get_inactive_time(nullptr);
+            inactive_ms = lv_display_get_inactive_time(nullptr);
             lvgl::unlock();
         }