fix(infra): make MEILI_URL configurable; rename Discover → Catalogue in nav

- docker-compose.yml: MEILI_URL in x-infra-env was hardcoded to the local
  meilisearch container, ignoring the Doppler MEILI_URL secret entirely.
  Changed to "${MEILI_URL:-http://meilisearch:7700}" so prod reads from
  https://search.libnovel.cc while local dev keeps the container default.
- ui/+layout.svelte: rename nav + footer label from "Discover" to "Catalogue"
  (desktop nav, mobile menu, footer — all three occurrences).

docker-compose.yml

@@ -15,7 +15,7 @@ x-infra-env: &infra-env
   POCKETBASE_ADMIN_EMAIL: "${POCKETBASE_ADMIN_EMAIL}"
   POCKETBASE_ADMIN_PASSWORD: "${POCKETBASE_ADMIN_PASSWORD}"
   # Meilisearch
-  MEILI_URL: "http://meilisearch:7700"
+  MEILI_URL: "${MEILI_URL:-http://meilisearch:7700}"
   MEILI_API_KEY: "${MEILI_MASTER_KEY}"
   # Valkey
   VALKEY_ADDR: "valkey:6379"