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/McpSettings.cpp

@@ -11,7 +11,6 @@ static const auto LOGGER = Logger("McpSettings");
 constexpr auto* SETTINGS_FILE = "/data/service/mcp/settings.properties";
 
 constexpr auto* KEY_MCP_ENABLED = "mcpEnabled";
-constexpr auto* KEY_MCP_OVERRIDE_ENABLED = "mcpOverrideEnabled";
 
 bool load(McpSettings& settings) {
     std::map<std::string, std::string> map;
@@ -20,23 +19,16 @@ bool load(McpSettings& settings) {
     }
 
     auto mcp_enabled = map.find(KEY_MCP_ENABLED);
-    auto mcp_override = map.find(KEY_MCP_OVERRIDE_ENABLED);
-
     settings.mcpEnabled = (mcp_enabled != map.end())
         ? (mcp_enabled->second == "1" || mcp_enabled->second == "true")
         : false;
 
-    settings.mcpOverrideEnabled = (mcp_override != map.end())
-        ? (mcp_override->second == "1" || mcp_override->second == "true")
-        : true; // Default true
-
     return true;
 }
 
 McpSettings getDefault() {
     return McpSettings{
-        .mcpEnabled = false,
-        .mcpOverrideEnabled = true
+        .mcpEnabled = false
     };
 }
 
@@ -55,7 +47,6 @@ bool save(const McpSettings& settings) {
 
     std::map<std::string, std::string> map;
     map[KEY_MCP_ENABLED] = settings.mcpEnabled ? "true" : "false";
-    map[KEY_MCP_OVERRIDE_ENABLED] = settings.mcpOverrideEnabled ? "true" : "false";
 
     if (!file::savePropertiesFile(SETTINGS_FILE, map)) {
         LOGGER.error("Failed to save MCP settings to {}", SETTINGS_FILE);