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/themes/[id]/css/route.ts

@@ -0,0 +1,46 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getThemeCSS, getThemeRegistry } from '@/lib/admin/plugin-registry';
+import { logger } from '@/lib/logger';
+
+/**
+ * GET /api/admin/themes/[id]/css — Serve theme CSS to clients
+ */
+export async function GET(
+  _request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  try {
+    const { id } = await params;
+
+    // Validate ID format
+    if (!/^[a-z0-9][a-z0-9-]*[a-z0-9]$/.test(id)) {
+      return NextResponse.json({ error: 'Invalid theme ID' }, { status: 400 });
+    }
+
+    // Verify theme exists and is enabled
+    const registry = await getThemeRegistry();
+    const theme = registry.themes.find(t => t.id === id);
+    if (!theme) {
+      return NextResponse.json({ error: 'Theme not found' }, { status: 404 });
+    }
+    if (!theme.enabled) {
+      return NextResponse.json({ error: 'Theme is disabled' }, { status: 403 });
+    }
+
+    const css = await getThemeCSS(id);
+    if (!css) {
+      return NextResponse.json({ error: 'Theme CSS not found' }, { status: 404 });
+    }
+
+    return new NextResponse(css, {
+      headers: {
+        'Content-Type': 'text/css; charset=utf-8',
+        'Cache-Control': 'public, max-age=3600',
+        'X-Content-Type-Options': 'nosniff',
+      },
+    });
+  } catch (error) {
+    logger.error('Theme CSS serve error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}