feat: add plugin/theme disable gates and move policy controls to their admin pages

- Add `pluginsEnabled` and `themesEnabled` master feature gates to FeatureGates
- Move theme policy UI (default theme, built-in/admin theme toggles, user uploads toggle) from policy page to themes admin page
- Add plugin policy UI (plugins enabled toggle) to plugins admin page
- Remove theme policy section and plugin/theme gates from policy page (with note directing to respective pages)
- Hide Themes and Plugins settings tabs when their feature gate is disabled
- Fix dark mode visibility of all admin toggle switches (bg-white → bg-background, increase off-state track opacity)

app/admin/settings/page.tsx

@@ -197,9 +197,9 @@ function ToggleSetting({ label, description, configKey, value, source, onChange,
       <div className="flex items-center gap-2">
         <button
           onClick={() => onChange(configKey, !value)}
-          className={`relative inline-flex h-5 w-9 items-center rounded-full transition-colors ${value ? 'bg-primary' : 'bg-muted-foreground/30'}`}
+          className={`relative inline-flex h-5 w-9 items-center rounded-full transition-colors ${value ? 'bg-primary' : 'bg-muted-foreground/25 dark:bg-muted-foreground/50'}`}
         >
-          <span className={`inline-block h-3.5 w-3.5 transform rounded-full bg-white shadow transition-transform ${value ? 'translate-x-[18px]' : 'translate-x-[3px]'}`} />
+          <span className={`inline-block h-3.5 w-3.5 transform rounded-full bg-background shadow transition-transform ${value ? 'translate-x-[18px]' : 'translate-x-[3px]'}`} />
         </button>
         {source === 'admin' && (
           <button onClick={() => onRevert(configKey)} className="text-muted-foreground hover:text-foreground" title="Revert to default">