Add dark color scheme, backlight timeout, and L/R swap feature

- Invert UI colors: black background with white text, yellow highlights
- Add 30-second backlight timeout with auto-wake on button press
- Add Swap L/R menu option with checkbox to swap audio channels
- Update volume page styling (yellow bar, full-screen container)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

main/system.c

@@ -23,6 +23,7 @@ void system_init(void)
     _systemState.primaryAxis = PRIMARY_AXIS;
     _systemState.pairedDeviceCount = 0;
     _systemState.isCharging = false;
+    _systemState.swapLR = false;
 
 	_systemEvent = xEventGroupCreate();
 
@@ -74,6 +75,31 @@ bool system_getChargeStatus(void)
     return charging;
 }
 
+void system_setSwapLR(bool swap)
+{
+    xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);
+    _systemState.swapLR = swap;
+    xSemaphoreGive(_eventManager.mutex);
+    ESP_LOGI("system", "Swap L/R: %s", swap ? "ON" : "OFF");
+}
+
+bool system_getSwapLR(void)
+{
+    bool swap;
+    xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);
+    swap = _systemState.swapLR;
+    xSemaphoreGive(_eventManager.mutex);
+    return swap;
+}
+
+void system_toggleSwapLR(void)
+{
+    xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);
+    _systemState.swapLR = !_systemState.swapLR;
+    ESP_LOGI("system", "Swap L/R toggled: %s", _systemState.swapLR ? "ON" : "OFF");
+    xSemaphoreGive(_eventManager.mutex);
+}
+
 void system_setZeroAngle(void)
 {
     xSemaphoreTake(_eventManager.mutex, portMAX_DELAY);