Closes#250: kill confirmation/force-kill (dialogs/KillDialog), the ?
shortcuts overlay (dialogs/HelpOverlay), the general-purpose right-click
menu (common/ContextMenu, closes on scroll/resize unlike the sidebar's
own), the mobile drawer with swipe support (layout/Drawer), and the
visual-viewport hook (hooks/useViewport). Each is a standalone, tested
port of its vanilla-DOM source; wiring into the shell is a later issue.
