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/service/displayidle/DisplayIdle.cpp

@@ -7,6 +7,7 @@
 #include "MatrixRainScreensaver.h"
 #include "MystifyScreensaver.h"
 #include "StackChanScreensaver.h"
+#include "McpScreensaver.h"
 
 #include <Tactility/Logger.h>
 #include <Tactility/CoreDefines.h>
@@ -103,6 +104,9 @@ void DisplayIdleService::activateScreensaver() {
         case settings::display::ScreensaverType::StackChan:
             screensaver = std::make_unique<StackChanScreensaver>();
             break;
+        case settings::display::ScreensaverType::McpScreen:
+            screensaver = std::make_unique<McpScreensaver>();
+            break;
         case settings::display::ScreensaverType::None:
         default:
             // Just black screen, no animated screensaver