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/Tactility.cpp

@@ -119,7 +119,6 @@ namespace app {
     namespace crashdiagnostics { extern const AppManifest manifest; }
     namespace webserversettings { extern const AppManifest manifest; }
     namespace mcpsettings { extern const AppManifest manifest; }
-    namespace mcpoverride { extern const AppManifest manifest; }
 #if CONFIG_TT_TDECK_WORKAROUND == 1
     namespace keyboardsettings { extern const AppManifest manifest; } // T-Deck only for now
     namespace trackballsettings { extern const AppManifest manifest; } // T-Deck only for now
@@ -170,7 +169,6 @@ static void registerInternalApps() {
     addAppManifest(app::apwebserver::manifest);
     addAppManifest(app::webserversettings::manifest);
     addAppManifest(app::mcpsettings::manifest);
-    addAppManifest(app::mcpoverride::manifest);
     addAppManifest(app::crashdiagnostics::manifest);
     addAppManifest(app::development::manifest);
 #if defined(CONFIG_TT_TDECK_WORKAROUND)