Fix service workers

site/.env.example

@@ -5,6 +5,11 @@ PUBLIC_SITE_URL=https://example.com
 PUBLIC_UMAMI_SCRIPT_URL=https://analytics.example.com/script.js
 PUBLIC_UMAMI_WEBSITE_ID=00000000-0000-0000-0000-000000000000
 
+# Service worker toggle (public, build-time). Use to enable in prod/staging.
+# - In `npm run dev`, service workers are not registered regardless.
+# - In `npm run build`/`preview`, SW is registered only if this is "true".
+PUBLIC_ENABLE_SW=true
+
 # Content ingestion configuration (used by scripts)
 YOUTUBE_CHANNEL_ID=UCxxxxxxxxxxxxxxxxxxxxxx
 YOUTUBE_API_KEY=