fix: deployment readiness — correct registry/cloud URLs, fix install.sh

- .env.example: set REGISTRY=registry.bonamin.gr, CLOUD_URL=https://xenia-admin.bonamin.gr, DATA_PATH=/opt/xenia/data
- install.sh: auto-create .env from example, prompt for SITE_ID/SITE_KEY/SECRET_KEY,
  clarify DNS subdomain requirements, add backend API proxy block to nginx config

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

.env.example

@@ -1,14 +1,14 @@
 # Registry
-REGISTRY=registry.yourdomain.com
+REGISTRY=registry.bonamin.gr
 VERSION=1.0.0
 
-# Backend runtime secrets (get SITE_ID and SITE_KEY from sysadmin panel)
+# Backend runtime secrets (get SITE_ID and SITE_KEY from the sysadmin panel)
 SITE_ID=your-site-id
 SITE_KEY=your-site-key
-CLOUD_URL=https://api.yourdomain.com
+CLOUD_URL=https://xenia-admin.bonamin.gr
 SECRET_KEY=generate-with-openssl-rand-hex-32
 LICENSE_GRACE_HOURS=24
 
-# Volumes — absolute paths recommended on client machines
-DATA_PATH=/home/user/appdata/pos/data
-LOGO_PATH=/home/user/appdata/pos/logo.png
+# Volumes — absolute paths on the client machine
+DATA_PATH=/opt/xenia/data
+LOGO_PATH=/opt/xenia/logo.png