feat(domain): separate POS to pos.nexusautoparts.com.mx subdomain

- nexusautoparts.com.mx -> Dashboard/Landing (port 5000)
- pos.nexusautoparts.com.mx -> POS (port 5001) with static assets proxy
- admin.nexusautoparts.com.mx -> Dashboard (port 5000)
- Update mobile app configs to point to pos.nexusautoparts.com.mx
- Update Caddy docs with new subdomain layout

pos/capacitor.config.json

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

pos/mobile/README.md

@@ -8,7 +8,7 @@
 ## Architecture
 
 The Capacitor app loads the POS from the remote server at
-`https://nexusautoparts.com.mx/pos`. This means:
+`https://pos.nexusautoparts.com.mx/pos`. This means:
 - The app requires internet on first load.
 - The PWA service worker handles offline caching after that.
 - No HTML/JS/CSS is bundled into the native binary.