Switch data volume from bind mount to named Docker volume

Bind mount (./data:/app/data) was relative to the working directory,
so data was lost whenever the project was re-cloned or compose was run
from a different path. A named volume is managed by Docker and persists
across container rebuilds and image pulls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -5,5 +5,8 @@ services:
     ports:
       - "3000:3000"
     volumes:
-      - ./data:/app/data
+      - spelling-data:/app/data
     restart: unless-stopped
+
+volumes:
+  spelling-data: