topbar.js binds click handlers to every .dark-toggle, including the
settings one; settings.js then also wired a delegation that re-clicked
the topbar button, so each click flipped dark mode twice (net no-op).
Mark buttons as wired in topbar.js and only add a fallback handler in
settings.js when topbar.js did not run.
CSS: center the toggle horizontally under the Settings title with auto
margins, matching .settings-body padding.
Closes#197