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/api/admin/policy/route.ts

@@ -43,6 +43,9 @@ export async function PUT(request: NextRequest) {
     if (policy.features && typeof policy.features !== 'object') {
       return NextResponse.json({ error: 'features must be an object' }, { status: 400 });
     }
+    if (policy.themePolicy && typeof policy.themePolicy !== 'object') {
+      return NextResponse.json({ error: 'themePolicy must be an object' }, { status: 400 });
+    }
 
     await configManager.setPolicy(policy);
     await auditLog('policy.update', { restrictionCount: Object.keys(policy.restrictions || {}).length }, ip);