TerminalWebView wraps a WebView around the bundled xterm.js page, with
a JS bridge (maestroKey/maestroToggleMod/maestroFocus) the native key
bar drives; onModifierConsumed reports back when the page's own JS
folds an armed Ctrl/Alt into a soft-keyboard keystroke, so the native
toggle stays in sync. TerminalScreen composes the WebView with the key
bar, showing the bar only while WindowInsets.isImeVisible.