fix(mcp): configure mcp_client allowlist on project switch

Previously mcp_client.configure() was only called during ai_client.send()
which meant GUI buttons (Slices/Inspect) couldn't access files when
project was switched to an external project like gencpp. Now _switch_project
reconfigures mcp_client with the new project's root and file_items.

src/gui_2.py

@@ -2909,7 +2909,10 @@ class App:
       self.ui_editing_slices_file = f_item
       f_path = f_item.path if hasattr(f_item, "path") else str(f_item)
       self.text_viewer_title = f"Slices: {f_path}"
-      self.text_viewer_content = f_item.content or ""
+      try:
+       self.text_viewer_content = mcp_client.read_file(f_path)
+      except Exception as e:
+       self.text_viewer_content = f"Error reading file: {e}"
       self.text_viewer_type = 'cpp' if f_path.endswith(('.cpp', '.hpp', '.h')) else 'python' if f_path.endswith('.py') else 'text'
       self.show_text_viewer = True