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/settings.js

@@ -91,11 +91,15 @@
     if (dt) dt.setAttribute('aria-pressed', s.dark ? 'true' : 'false');
   }
   sync();
-  // the topbar script runs before this button exists, so wire it here
+  // topbar.js already binds every .dark-toggle (deferred, so the settings
+  // button exists by then). Only wire here if it somehow did not run,
+  // otherwise the button would toggle twice per click and never change (#197).
   var dt = document.getElementById('settings-dark-toggle');
-  dt.addEventListener('click', function () {
-    var btns = document.querySelectorAll('.topbar .dark-toggle');
-    if (btns.length) btns[0].click(); else document.dispatchEvent(new CustomEvent('palette-darkchange'));
-  });
+  if (dt && !dt.dataset.darkWired) {
+    dt.addEventListener('click', function () {
+      var btns = document.querySelectorAll('.topbar .dark-toggle');
+      if (btns.length) btns[0].click(); else document.dispatchEvent(new CustomEvent('palette-darkchange'));
+    });
+  }
   document.addEventListener('palette-darkchange', sync);
 })();