fix: serialize TTY writes and fix terminal resize/session edge cases

[KUASWoodyLIN]

• Queue stdin into a single in-flight write to stop keystrokes from reordering under fast typing
• Debounce remote resize (100ms) and skip resizing while the container is hidden or zero-sized
• Repaint the xterm viewport's hardcoded black background with the theme color
• Copy selected text automatically instead of via Ctrl+Shift+C, which collided with the browser's inspect-element shortcut
• Require Enter to start a new session after one ends instead of auto-reconnecting, and skip the retry prompt when the container never produced any output (unsupported interactive shell)