build(docker): upgrade pgvector image to pg18

Upgrades all references of pgvector/pgvector from pg16 to pg18,
including docker-compose files and e2e testing environments.
Updates the development docker-compose to mount the postgres volume
at /var/lib/postgresql to align with pg18+ directory requirements.

Also adds `shrink-0` to WhatsNewDialog header and footer to resolve
a layout overflow glitch.

docker-compose.dev.yml

@@ -3,7 +3,7 @@ name: bookorbit-dev
 services:
   postgres:
     container_name: bookorbit-dev-db
-    image: pgvector/pgvector:pg16
+    image: pgvector/pgvector:pg18
     restart: unless-stopped
     environment:
       POSTGRES_USER: ${DEV_POSTGRES_USER:-bookorbit}
@@ -13,7 +13,7 @@ services:
     ports:
       - "${DEV_POSTGRES_PORT:-5432}:5432"
     volumes:
-      - postgres_data:/var/lib/postgresql/data
+      - postgres_data:/var/lib/postgresql
       - ./docker/postgres/init.sql:/docker-entrypoint-initdb.d/init.sql
     healthcheck:
       test: