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/app.css

@@ -255,7 +255,13 @@ body {
 .theme-grid { display: flex; flex-wrap: wrap; gap: 14px; margin-top: 14px; }
 
 /* #161: more breathing room between settings sections */
-#settings-dark-toggle { margin-bottom: 20px; }
+#settings-dark-toggle {
+  margin-bottom: 20px;
+  margin-left: auto;
+  margin-right: auto;
+  display: flex;
+  justify-content: center;
+}
 .settings-body h3 { margin-top: 28px; }
 .settings-body .theme-grid { margin-bottom: 8px; }
 .theme-card {