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/MystifyScreensaver.h

@@ -0,0 +1,72 @@
+#pragma once
+
+#ifdef ESP_PLATFORM
+
+#include "Screensaver.h"
+#include <array>
+#include <cstdint>
+
+namespace tt::service::displayidle {
+
+class MystifyScreensaver final : public Screensaver {
+public:
+    static constexpr int NUM_POLYGONS = 2;
+    static constexpr int NUM_VERTICES = 4;
+    static constexpr int TRAIL_LENGTH = 8;
+    static constexpr int COLOR_CHANGE_INTERVAL = 200;  // Frames between color changes (~10s at 20fps)
+
+    ~MystifyScreensaver() override = default;
+    MystifyScreensaver() = default;
+    MystifyScreensaver(const MystifyScreensaver&) = delete;
+    MystifyScreensaver& operator=(const MystifyScreensaver&) = delete;
+    MystifyScreensaver(MystifyScreensaver&&) = delete;
+    MystifyScreensaver& operator=(MystifyScreensaver&&) = delete;
+
+    void start(lv_obj_t* overlay, lv_coord_t screenW, lv_coord_t screenH) override;
+    void stop() override;
+    void update(lv_coord_t screenW, lv_coord_t screenH) override;
+
+private:
+    // Use floats for smooth sub-pixel movement
+    struct Vertex {
+        float x = 0;
+        float y = 0;
+        float dx = 0;
+        float dy = 0;
+    };
+
+    struct Polygon {
+        std::array<Vertex, NUM_VERTICES> vertices;
+        // History: [trail_index][vertex_index] = {x, y}
+        std::array<std::array<lv_point_precise_t, NUM_VERTICES>, TRAIL_LENGTH> history;
+        // Line objects: [trail_index][edge_index]
+        std::array<std::array<lv_obj_t*, NUM_VERTICES>, TRAIL_LENGTH> lines{};
+        // Persistent point arrays for each line (required by lv_line_set_points)
+        std::array<std::array<std::array<lv_point_precise_t, 2>, NUM_VERTICES>, TRAIL_LENGTH> linePoints;
+        lv_color_t color = {};
+        int historyHead = 0;
+        bool historyFull = false;
+        int colorChangeCounter = 0;
+    };
+
+    // Vibrant color pool for random selection
+    static constexpr std::array<uint32_t, 8> COLOR_POOL = {
+        0xFF00FF,  // Magenta
+        0x00FFFF,  // Cyan
+        0xFFFF00,  // Yellow
+        0xFF6600,  // Orange
+        0x00FF66,  // Spring green
+        0x6600FF,  // Purple
+        0xFF0066,  // Hot pink
+        0x66FF00,  // Lime
+    };
+
+    std::array<Polygon, NUM_POLYGONS> polygons_;
+
+    void initPolygon(Polygon& polygon, lv_obj_t* parent, lv_coord_t screenW, lv_coord_t screenH);
+    void updateLineColors(Polygon& polygon);
+};
+
+} // namespace tt::service::displayidle
+
+#endif // ESP_PLATFORM