Add MCP bridge for external tool integration

Embedded JSON-RPC server over named pipes (rcx-mcp) enabling external
tools like Claude Code to inspect and manipulate the node tree, read/write
hex data, switch sources, and trigger UI actions. Includes stdio adapter
(rcx-mcp-stdio) for stdin/stdout transport. Server is stopped by default;
user starts via File > Start MCP Server.

Also extracts MainWindow class declaration to mainwindow.h and improves
type selector popup Esc button styling.

src/typeselectorpopup.h

@@ -39,7 +39,7 @@ protected:
 
 private:
     QLabel*           m_titleLabel   = nullptr;
-    QLabel*           m_escLabel     = nullptr;
+    QToolButton*      m_escLabel     = nullptr;
     QToolButton*      m_createBtn    = nullptr;
     QLineEdit*        m_filterEdit   = nullptr;
     QListView*        m_listView     = nullptr;