Standardize SurClaw ports away from defaults

- Change the API default listen port from 3000 to 3100 and include the Docker frontend origin in default CORS.

- Point Vite's default API proxy, Docker API container port, and Nginx upstream to 3100.

- Keep Docker host ports on 4002 for web, 3002 for API, and 5433 for PostgreSQL.

- Update environment examples and documentation to remove stale localhost:3000 guidance.

.env.example

@@ -1,9 +1,9 @@
 # Backend API development defaults.
-API_PORT=3000
-CORS_ORIGIN="http://localhost:3001"
+API_PORT=3100
+CORS_ORIGIN="http://localhost:3001,http://localhost:4002"
 DATABASE_URL="postgresql://surclaw:surclaw_dev_password@localhost:5433/surclaw?schema=public"
 SESSION_SECRET="change-me-in-production"
 SESSION_COOKIE_SECURE="false"
 FILE_STORAGE_DIR="./uploads"
-VITE_API_PROXY_TARGET="http://localhost:3000"
+VITE_API_PROXY_TARGET="http://localhost:3100"
 VITE_ENABLE_LOCAL_FALLBACK="true"