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/settings/DisplaySettings.cpp

@@ -78,6 +78,8 @@ static std::string toString(ScreensaverType type) {
             return "MatrixRain";
         case StackChan:
             return "StackChan";
+        case McpScreen:
+            return "McpScreen";
         default:
             std::unreachable();
     }
@@ -99,6 +101,9 @@ static bool fromString(const std::string& str, ScreensaverType& type) {
     } else if (str == "StackChan") {
         type = ScreensaverType::StackChan;
         return true;
+    } else if (str == "McpScreen") {
+        type = ScreensaverType::McpScreen;
+        return true;
     } else {
         return false;
     }