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 -->

Tactility/Source/service/displayidle/Screensaver.h

@@ -0,0 +1,48 @@
+#pragma once
+#ifdef ESP_PLATFORM
+
+#include <lvgl.h>
+
+namespace tt::service::displayidle {
+
+/**
+ * Base class for screensaver implementations.
+ * Screensavers are responsible for creating their visual elements
+ * on the provided overlay and updating them each tick.
+ */
+class Screensaver {
+public:
+    Screensaver() = default;
+    virtual ~Screensaver() = default;
+    Screensaver(const Screensaver&) = delete;
+    Screensaver& operator=(const Screensaver&) = delete;
+    Screensaver(Screensaver&&) = delete;
+    Screensaver& operator=(Screensaver&&) = delete;
+
+    /**
+     * Start the screensaver, creating visual elements on the overlay.
+     * @param overlay The parent LVGL object to create elements on
+     * @param screenW Screen width in pixels
+     * @param screenH Screen height in pixels
+     */
+    virtual void start(lv_obj_t* overlay, lv_coord_t screenW, lv_coord_t screenH) = 0;
+
+    /**
+     * Stop the screensaver, cleaning up any internal state.
+     * Note: LVGL objects are deleted by the parent overlay, but any
+     * internal references should be cleared here.
+     */
+    virtual void stop() = 0;
+
+    /**
+     * Update the screensaver animation for one frame.
+     * Called periodically while the screensaver is active.
+     * @param screenW Screen width in pixels
+     * @param screenH Screen height in pixels
+     */
+    virtual void update(lv_coord_t screenW, lv_coord_t screenH) = 0;
+};
+
+} // namespace tt::service::displayidle
+
+#endif // ESP_PLATFORM