chore(domain): migrate to nexusautoparts.com.mx

- Update Nginx config:
  - nexusautoparts.com.mx -> POS (port 5001)
  - admin.nexusautoparts.com.mx -> Dashboard (port 5000)
  - nexus.consultoria-as.com -> legacy redirect (dashboard)
  - Add redirect / -> /pos/login for main domain
- Update domain references in code:
  - capacitor.config.json (mobile apps)
  - pos/mobile/README.md
  - pos/config.py SMTP_FROM
- Add Caddy config docs for reverse proxy VM (192.168.10.74)

pos/capacitor.config.json

@@ -3,7 +3,7 @@
   "appName": "Nexus POS",
   "webDir": "www",
   "server": {
-    "url": "https://nexus.consultoria-as.com/pos",
+    "url": "https://nexusautoparts.com.mx/pos",
     "cleartext": true
   },
   "plugins": {