feat: implement MCP drawing as screensaver instead of standalone app

- Add ScreensaverType::McpScreen to DisplaySettings enum, toString/fromString
- Add 'MCP Screen' option to the Display settings screensaver dropdown
- New McpScreensaver (Screensaver subclass): creates full-screen lv_canvas on
  the overlay, allocates framebuffer in SPIRAM, registers canvas pointer in
  McpSystemState, shows 'Waiting for LLM...' until first draw command arrives
- Wire McpScreensaver into DisplayIdle::activateScreensaver() switch
- McpSystem::ensureOverrideScreen() now calls displayidle::findService()->
  startScreensaver() instead of launching McpOverrideApp — canvas appears
  automatically on any LLM draw command
- Remove McpOverrideApp (replaced by screensaver), deregister from Tactility.cpp
- Remove mcpOverrideEnabled from McpSettings — no longer needed
- Simplify McpSettingsApp: remove Screen Override toggle, update info text to
  direct users to Settings -> Display -> Screensaver -> MCP Screen

Tactility/Source/mcp/McpSystem.cpp

@@ -6,6 +6,7 @@
 #include <Tactility/file/File.h>
 #include <Tactility/app/App.h>
 #include <Tactility/Logger.h>
+#include <Tactility/service/displayidle/DisplayIdleService.h>
 
 #include <freertos/FreeRTOS.h>
 #include <freertos/task.h>
@@ -76,19 +77,21 @@ static bool display_ready(McpSystemState& state) {
 
 static bool ensureOverrideScreen() {
     auto& state = getState();
-    auto mcpSettings = settings::mcp::loadOrGetDefault();
-    if (!mcpSettings.mcpOverrideEnabled) {
-        return false;
-    }
     if (state.drawArea == nullptr) {
-        LOGGER.info("MCP Screen Override triggered: starting McpOverrideApp");
-        tt::app::start("one.tactility.mcpscreen");
-        // Wait up to 500ms for UI to initialize
-        for (int i = 0; i < 10; ++i) {
-            vTaskDelay(pdMS_TO_TICKS(50));
-            if (state.drawArea != nullptr) {
-                break;
+        // Activate the MCP Screen screensaver via DisplayIdle
+        auto idleService = service::displayidle::findService();
+        if (idleService) {
+            LOGGER.info("MCP draw triggered: starting MCP screensaver");
+            idleService->startScreensaver();
+            // Wait up to 500ms for the canvas to be registered
+            for (int i = 0; i < 10; ++i) {
+                vTaskDelay(pdMS_TO_TICKS(50));
+                if (state.drawArea != nullptr) {
+                    break;
+                }
             }
+        } else {
+            LOGGER.warn("DisplayIdle service not found; cannot activate MCP screensaver");
         }
     }
     return state.drawArea != nullptr;