Share session cookies across www and app subdomains

Groundwork for moving the app to app.qrmaster.net: the session has to survive the
host change from www.qrmaster.net to app.qrmaster.net.

- Add COOKIE_DOMAIN and apply it to the auth, CSRF, attribution and OAuth flow
  cookies. Honoured only in production, because browsers reject dotted domains on
  localhost - a prod .env copied into a dev environment would otherwise break
  every login instead of just ignoring the value.
- Expire both the host-only and the domain-scoped variant on logout. Next's
  ResponseCookies is keyed by cookie name and rewrites the entire set-cookie
  header from its internal map on every set(), so the two variants must be
  appended manually - otherwise one overwrites the other and the surviving stale
  cookie keeps the user signed in.
- Pass COOKIE_DOMAIN as both build arg and runtime env: process.env is inlined
  into the Edge middleware bundle, so a runtime-only value would leave the
  middleware and the route handlers disagreeing about the cookie scope.

No behaviour change while COOKIE_DOMAIN is unset.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.yml

@@ -46,6 +46,7 @@ services:
       args:
         NEXT_PUBLIC_UMAMI_SRC: ${NEXT_PUBLIC_UMAMI_SRC:-}
         NEXT_PUBLIC_UMAMI_ID: ${NEXT_PUBLIC_UMAMI_ID:-}
+        COOKIE_DOMAIN: ${COOKIE_DOMAIN:-}
     container_name: qrmaster-web
     restart: unless-stopped
     environment:
@@ -56,6 +57,7 @@ services:
       NEXTAUTH_URL: ${NEXTAUTH_URL}
       NEXTAUTH_SECRET: ${NEXTAUTH_SECRET}
       NEXT_PUBLIC_APP_URL: ${NEXT_PUBLIC_APP_URL:-http://localhost:3050}
+      COOKIE_DOMAIN: ${COOKIE_DOMAIN:-}
       INTERNAL_API_SECRET: ${INTERNAL_API_SECRET}
       TIKTOK_CLIENT_KEY: ${TIKTOK_CLIENT_KEY:-}
       TIKTOK_CLIENT_SECRET: ${TIKTOK_CLIENT_SECRET:-}