Change frontend port to 3001 and fix REACT_APP_API_URL as build arg

REACT_APP_API_URL is baked into the JS bundle at build time, so it
must be passed as a build arg, not a runtime environment variable.
Port changed from 3000 to 3001 to avoid conflict with Gitea.

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

btc-portfolio/docker-compose.yml

@@ -10,11 +10,12 @@ services:
     restart: unless-stopped
 
   frontend:
-    build: ./frontend
-    ports:
-      - "3000:3000"
-    environment:
+    build:
+      context: ./frontend
+      args:
         - REACT_APP_API_URL=http://localhost:8000
+    ports:
+      - "3001:3000"
     depends_on:
       - backend
     restart: unless-stopped