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/layout.tsx

@@ -12,6 +12,8 @@ import {
   ScrollText,
   LogOut,
   KeyRound,
+  Puzzle,
+  SwatchBook,
 } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import { useConfig } from '@/hooks/use-config';
@@ -23,6 +25,8 @@ const NAV_ITEMS = [
   { href: '/admin/branding', label: 'Branding', icon: Palette },
   { href: '/admin/auth', label: 'Authentication', icon: Shield },
   { href: '/admin/policy', label: 'Policy', icon: Scale },
+  { href: '/admin/plugins', label: 'Plugins', icon: Puzzle },
+  { href: '/admin/themes', label: 'Themes', icon: SwatchBook },
   { href: '/admin/logs', label: 'Audit Log', icon: ScrollText },
 ];