fix(docker): SPA copy path + simpler external network ref

Two bugs that surfaced on first real deploy:

1. SPA was COPYed to /app/apps/api/public but server.ts looks at
   /app/apps/web/dist (relative to apps/api/dist via fileURLToPath).
   Static handler returned 404 for /. Move the COPY to match.

2. The aliased external network (`umbrel_main` -> name:
   umbrel_main_network) wasn't attaching in Portainer Stacks → DNS
   lookup of datum_datum_1 fails with ENOTFOUND. Use the network's
   real name directly as the alias to avoid any Portainer indirection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -39,12 +39,9 @@ services:
       retries: 3
       start_period: 15s
     networks:
-      - umbrel_main
+      - umbrel_main_network
       - default
 
 networks:
-  umbrel_main:
+  umbrel_main_network:
     external: true
-    # Datum container on this Umbrel sits on `umbrel_main_network` (10.21.0.0/16).
-    # Confirmed via `docker network ls` on the Umbrel host.
-    name: umbrel_main_network