fix(mcp): implement startMcpScreensaver - missing method caused linker error

- Was declared in DisplayIdleService.h but not defined in clean refactored branch
- Implementation taken from original 8970143f: locks LVGL, checks overlay, ensures backlight, creates McpScreensaver
- Fixes build of feature/local-all-rebased on es3c28p

Tactility/Source/service/displayidle/DisplayIdle.cpp

@@ -9,11 +9,11 @@
 #include "StackChanScreensaver.h"
 #include "McpScreensaver.h"
 
-#include <tactility/log.h>
 #include <Tactility/CoreDefines.h>
 #include <Tactility/hal/display/DisplayDevice.h>
 #include <Tactility/hal/power/PowerDevice.h>
 #include <Tactility/lvgl/LvglSync.h>
+#include <Tactility/mcp/McpSystem.h>
 #include <Tactility/service/ServiceContext.h>
 #include <Tactility/service/ServiceManifest.h>
 #include <Tactility/service/ServiceRegistration.h>
@@ -284,6 +284,61 @@ bool DisplayIdleService::isScreensaverActive() const {
     return screensaverOverlay != nullptr;
 }
 
+void DisplayIdleService::startMcpScreensaver() {
+    if (!lvgl::lock(200)) {
+        LOG_W(TAG, "startMcpScreensaver: failed to acquire LVGL lock");
+        return;
+    }
+
+    if (screensaverOverlay != nullptr) {
+        // Screensaver already active — if drawArea is registered we're done,
+        // otherwise stop the current one so we can replace it with McpScreensaver.
+        const auto& mcpState = mcp::getState();
+        if (mcpState.drawArea != nullptr) {
+            lvgl::unlock();
+            return; // McpScreensaver already running
+        }
+        // Wrong screensaver type active — tear it down first
+        if (screensaver) {
+            screensaver->stop();
+            screensaver.reset();
+        }
+        lv_obj_delete(screensaverOverlay);
+        screensaverOverlay = nullptr;
+    }
+
+    screensaverActiveCounter = 0;
+    backlightOff = false;
+
+    // Ensure backlight is active if the display supports it
+    auto display = getDisplay();
+    if (display != nullptr && display->supportsBacklightDuty()) {
+        uint8_t duty = cachedDisplaySettings.backlightDuty;
+        if (duty == 0) duty = 255; // ensure visible if settings not loaded / default
+        display->setBacklightDuty(duty);
+    }
+
+    lv_coord_t screenW = lv_display_get_horizontal_resolution(nullptr);
+    lv_coord_t screenH = lv_display_get_vertical_resolution(nullptr);
+
+    lv_obj_t* top = lv_layer_top();
+    screensaverOverlay = lv_obj_create(top);
+    lv_obj_remove_style_all(screensaverOverlay);
+    lv_obj_set_size(screensaverOverlay, LV_PCT(100), LV_PCT(100));
+    lv_obj_set_pos(screensaverOverlay, 0, 0);
+    lv_obj_set_style_bg_color(screensaverOverlay, lv_color_black(), 0);
+    lv_obj_set_style_bg_opa(screensaverOverlay, LV_OPA_COVER, 0);
+    lv_obj_add_flag(screensaverOverlay, LV_OBJ_FLAG_CLICKABLE);
+    lv_obj_add_event_cb(screensaverOverlay, stopScreensaverCb, LV_EVENT_CLICKED, this);
+
+    screensaver = std::make_unique<McpScreensaver>();
+    screensaver->start(screensaverOverlay, screenW, screenH);
+
+    lvgl::unlock();
+    displayDimmed = true;
+    LOG_I(TAG, "MCP screensaver activated");
+}
+
 void DisplayIdleService::reloadSettings() {
     // Set flag for thread-safe reload - actual reload happens in tick()
     settingsReloadRequested.store(true, std::memory_order_release);