Robust server lifecycle and debug preview

Add cross-platform process and port management for the Hexo server: import execFile, implement killProcessTree, getListeningPidsOnPort and killPortListeners to cleanly terminate server processes and free port 4000. Adjust server start to spawn differently on Windows (cmd.exe) vs POSIX (detached process), and improve server stop to kill process tree, clear listeners, and return accurate alreadyStopped status. In the renderer, add DEBUG_SERVER_URL and waitForDebugServerReady so the preview iframe only navigates once the server responds, improving reliability of the debug preview and status updates.