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>

Dockerfile

@@ -42,6 +42,11 @@ ARG NEXT_PUBLIC_UMAMI_SRC=""
 ARG NEXT_PUBLIC_UMAMI_ID=""
 ENV NEXT_PUBLIC_UMAMI_SRC=$NEXT_PUBLIC_UMAMI_SRC
 ENV NEXT_PUBLIC_UMAMI_ID=$NEXT_PUBLIC_UMAMI_ID
+# Shared session cookie across www.* and app.*. Needed at build time too: 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.
+ARG COOKIE_DOMAIN=""
+ENV COOKIE_DOMAIN=$COOKIE_DOMAIN
 RUN npx prisma generate
 RUN npm run build