library: proxy URL overrides, restore Quick Add header links

- OPAC_URL / STAFF_URL let Quick Add build correct links behind a reverse proxy
- restore the Catalogue/Staff header links (lost when the init block was
  rewritten) and verify them in the browser test

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wi3ZSot4XbUViwRFgLpRf

library/docker-compose.yml

@@ -64,6 +64,8 @@ services:
       KOHA_LIBRARY: ${LIBRARY_CODE:-HOME}
       OPAC_PORT: ${OPAC_PORT:-8080}
       STAFF_PORT: ${STAFF_PORT:-8081}
+      OPAC_URL: ${OPAC_URL:-}
+      STAFF_URL: ${STAFF_URL:-}
       QUICKADD_SOURCES: ${QUICKADD_SOURCES:-z3950:lx2.loc.gov:210/LCDB,openlibrary}
     ports:
       - "${BIND_ADDR:-0.0.0.0}:${QUICKADD_PORT:-8082}:8082"