Fix settings dark mode toggle double-binding and center it (#197)

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

internal/web/static/topbar.js

@@ -31,6 +31,9 @@
     apply();
     sync(Array.prototype.slice.call(btns));
     btns.forEach(function (b) {
+      // mark as wired so settings.js does not add a second handler
+      // (#197: double-binding made the settings toggle flip twice = no-op)
+      b.dataset.darkWired = '1';
       b.addEventListener('click', function () {
         var s = state();
         var dark = !s.dark;