Add GTM (Google Tag Manager) infrastructure — sitewide, independent of GA4

Adds a new dd_gtm_container_id setting and sitewide GTM injection (head script + body noscript iframe), following the existing dd_ga4_measurement_id pattern for the admin field/save logic. Deliberately NOT gated by is_dishdash_page() — GTM is meant to hold future tags (Meta Pixel, Google Ads conversion) whose page scope isn't known at deploy time, unlike GA4 which stays scoped to the ordering funnel. GA4/gtag logic fully untouched. Enables configuring new tracking pixels via GTM's dashboard without future code deploys, and sets up the pattern for other DishDash clients to add their own GTM container later.

See RELEASE.md for full details.