fix: spawn server with node on Windows to avoid Bun PTY incompatibility

[jesse23]

Summary

• On Windows, bun run webtty disconnected immediately in the browser while bunx webtty worked fine
• Root cause: two Bun-specific PTY paths both fail on Windows:
  • Bun.spawn({ terminal }) throws "terminal option is not supported on this platform"
  • @lydell/node-pty's ConPTY backend wraps named pipes using net.Socket({ fd }), which Bun's net.Socket does not support on Windows — resulting in pid=0 and ERR_SOCKET_CLOSED on the first write
• bunx webtty works because its #!/usr/bin/env node shim runs the server process with Node.js, where node-pty's ConPTY backend works correctly
• Fix: in startServer, when running on Windows under Bun, spawn the server with node instead of process.execPath — mirroring exactly what bunx does implicitly

Test plan

• bun run webtty on Windows — browser opens, terminal connects and stays connected
• bunx webtty on Windows — still works as before
• bun run webtty on macOS/Linux — still works (condition is Windows-only)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adjusts the CLI server startup path to avoid Bun-specific PTY incompatibilities on Windows by spawning the server under Node.js when the CLI itself is running under Bun on Windows.

Changes:

• Add Windows+Bun detection and select node as the server executable instead of process.execPath.
• Document the rationale in startServer with a Windows/Bun/node-pty ConPTY explanation.

Comments suppressed due to low confidence (1)

src/cli/http.ts:70

• Switching to serverExec = 'node' introduces a new failure mode where spawn can fail immediately (e.g. Node not on PATH) but startServer will still just poll until it times out and report "server did not start in time". Add explicit handling for child process spawn errors/early exit (and ideally a clearer message like "Node.js is required on Windows when running under Bun").

  const child = _spawn(serverExec, [serverEntry], {
    detached: true,
    stdio,
    env: { ...process.env, PORT: String(PORT) },
  });

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[jesse23]

Also addressed in fdade47: added a child.on('error', ...) handler so if node is not on PATH the process exits immediately with a clear message — "webtty: failed to start server: ... (Node.js must be on PATH when running webtty under Bun on Windows)" — instead of silently polling until the 10 s timeout.