fix(terminal): don't close pane WebSocket mid-handshake on teardown

[tdody]

Summary

Kills the dev-console warning WebSocket is closed before the connection is established logged from TerminalModal.tsx on (in dev) every modal open.

Cause

The terminal modal's cleanup called ws.close() unconditionally — including when the socket was still in CONNECTING. Closing a socket mid-handshake is exactly what makes the browser emit that warning.

React StrictMode (dev only) mounts every component twice — mount → unmount → remount — so opening a modal builds a throwaway WebSocket and tears it down before it finishes connecting, tripping the warning every time. A genuine fast dismiss (closing the modal before the socket connects) trips it in production too.

Fix

On teardown the cleanup now:

• detaches the socket's onmessage/onerror/onclose handlers (so a late frame can't write to the just-disposed terminal or re-arm reconnect logic), and
• if the socket is still CONNECTING, defers the close to onopen so it closes cleanly once connected; an already-open socket still closes immediately.

Tests (TDD)

• New TerminalModal — WebSocket teardown tests: a CONNECTING socket must not be closed directly on unmount (handlers detached, close deferred to onopen); an OPEN socket closes immediately. Watched the first fail against the old code, then pass.
• Added the real readyState constants (CONNECTING/OPEN/CLOSING/CLOSED) to the FakeWebSocket test double so the component's WebSocket.CONNECTING check resolves under test.
• Full frontend suite: 599 passing; typecheck clean.

Notes

• Independent of the backend FIFO-fd-leak fix (fix(pane-stream): release FIFO read fd on pane WebSocket disconnect #116).
• This does not address the separate xterm Cannot read properties of undefined (reading 'dimensions') error — that's an xterm 5.3 internal render-after-dispose race, dev-only (gone in production builds), and can't be reproduced under test since xterm is mocked. Tracked separately.

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