Without error handling, if node-pty require or pty.spawn throws (bad
permissions, missing cwd, stale server), the client gets no response and
the terminal hangs with a blinking cursor. Now sends an exit event back.
fitAddon.fit() was called synchronously on mount before the browser laid
out the container, sending 0-dimension cols/rows to the PTY. The shell
would hang without displaying a prompt. Now defers WS connection by 50ms,
re-fits after the shell reports ready, and shows an error if WS fails.
xterm.js frontend + node-pty WebSocket server spawning the user's real shell,
with drag-to-resize drawer, multi-terminal tabs, live theme sync, and URL state.
