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.

<sub>✏️ Tip: You can customize this high-level summary in your review settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Documentation/screensavers.md

@@ -0,0 +1,110 @@
+# DisplayIdle Service
+
+The DisplayIdle service manages screen timeout, screensavers, and backlight control for Tactility devices.
+
+## Features
+
+### Screen Timeout
+When enabled, the display will automatically dim after a configurable period of inactivity. Timeout options:
+- 15 seconds
+- 30 seconds
+- 1 minute
+- 2 minutes
+- 5 minutes
+- Never
+
+### Screensavers
+Four screensaver options are available:
+
+| Type | Description |
+|------|-------------|
+| **None** | Black screen only, backlight turns off immediately |
+| **Bouncing Balls** | Colored balls bouncing around the screen |
+| **Mystify** | Classic Windows-style polygon trails with color-changing effects |
+| **Matrix Rain** | Digital rain effect with terminal-style grid movement, 6-color gradient trails, glow effects, and random character flicker |
+
+### Auto-Off Feature
+After 5 minutes of screensaver activity, the screensaver animation stops and the backlight turns off completely to save power. Touching the screen restores normal operation.
+
+## Public API
+
+The service exposes a public header for external control:
+
+```cpp
+#include <Tactility/service/displayidle/DisplayIdleService.h>
+
+// Get service instance
+auto displayIdle = tt::service::displayidle::findService();
+
+// Force start screensaver immediately
+displayIdle->startScreensaver();
+
+// Force stop screensaver and restore backlight
+displayIdle->stopScreensaver();
+
+// Check if screensaver is currently active
+bool active = displayIdle->isScreensaverActive();
+
+// Reload settings (call after external settings changes)
+displayIdle->reloadSettings();
+```
+
+## Architecture
+
+### Files
+
+| File | Purpose |
+|------|---------|
+| `DisplayIdleService.h` | Public header with service interface |
+| `DisplayIdle.cpp` | Service implementation |
+| `Screensaver.h` | Base class for screensaver implementations |
+| `BouncingBallsScreensaver.h/cpp` | Bouncing balls screensaver |
+| `MystifyScreensaver.h/cpp` | Mystify polygon screensaver |
+| `MatrixRainScreensaver.h/cpp` | Matrix digital rain screensaver |
+
+### Screensaver Base Class
+
+All screensavers inherit from the `Screensaver` base class:
+
+```cpp
+class Screensaver {
+public:
+    virtual void start(lv_obj_t* overlay, lv_coord_t screenW, lv_coord_t screenH) = 0;
+    virtual void stop() = 0;
+    virtual void update(lv_coord_t screenW, lv_coord_t screenH) = 0;
+};
+```
+
+### Adding a New Screensaver
+
+1. Create header and implementation files inheriting from `Screensaver`
+2. Add enum value to `ScreensaverType` in `DisplaySettings.h` (before `Count` sentinel)
+3. Add string conversion in `DisplaySettings.cpp` (`toString` and `fromString`)
+4. Add dropdown option in `Display.cpp` (order must match enum order)
+5. Add case in `DisplayIdle.cpp` `activateScreensaver()` switch
+6. Include the new header in `DisplayIdle.cpp`
+
+**Note:** The `ScreensaverType::Count` sentinel must always be the last enum value - it's used for bounds checking in the UI.
+
+## Settings Integration
+
+Settings are stored in `/data/settings/display.properties` and managed through `DisplaySettings.h`:
+
+```cpp
+struct DisplaySettings {
+    Orientation orientation;
+    uint8_t gammaCurve;
+    uint8_t backlightDuty;
+    bool backlightTimeoutEnabled;
+    uint32_t backlightTimeoutMs;
+    ScreensaverType screensaverType;
+};
+```
+
+The Display app (`Display.cpp`) provides the UI for configuring these settings and notifies the DisplayIdle service when settings change via `reloadSettings()`.
+
+## Timing
+
+- Service tick interval: 50ms
+- Wake activity threshold: 100ms
+- Screensaver auto-off: 5 minutes (6000 ticks)