Fix dev compose: npm install on container startup

The ./server bind mount overwrites /app/server including node_modules
installed during the Docker build. Running npm install on startup
ensures deps are present after the volume mount.

Also reverts the node_modules named-volume workaround in favor of
this cleaner approach (requires node installed locally for non-Docker dev).

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

docker-compose.dev.yml

@@ -2,14 +2,9 @@ services:
   app:
     volumes:
       - ./server:/app/server
-      - server_node_modules:/app/server/node_modules
       - ./client:/app/client
-      - client_node_modules:/app/client/node_modules
     ports:
       - "5173:5173"
     environment:
       NODE_ENV: development
-
-volumes:
-  server_node_modules:
-  client_node_modules:
+    command: sh -c "npm install && node src/index.js"