Studio v2.0.3

Fixes
  - Homepage rendered a stale duplicate doc — query the singleton by _id
  - Extra margin between home chapters and the prefooter
  - Content crash on empty Portable Text body
  - EmbeddedVideo collapsing on mobile
  - Announcement bar with no link
  - Strict-mode build failure from optional css_hack
  - External-link detection + openInNewTab handling
  
Features
  - Stable IDs on header/footer menu items
  - Gated script-embed field; earlier analytics load
  
Other
  - Revalidation webhook secret decoupled from the Sanity read token