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

nginx/nexus-pos.conf

@@ -10,7 +10,7 @@ upstream nexus_quart {
     server 127.0.0.1:5002;
 }
 
-# ─── POS (primary domain) ───
+# ─── Landing page / Dashboard (primary domain) ───
 server {
     listen 80;
     server_name nexusautoparts.com.mx www.nexusautoparts.com.mx;
@@ -24,6 +24,30 @@ server {
     gzip_comp_level 6;
     gzip_types text/plain text/css text/xml application/json application/javascript application/rss+xml application/atom+xml image/svg+xml;
 
+    location / {
+        proxy_pass http://nexus_dashboard;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_read_timeout 300s;
+    }
+}
+
+# ─── POS (dedicated subdomain) ───
+server {
+    listen 80;
+    server_name pos.nexusautoparts.com.mx;
+
+    client_max_body_size 10M;
+
+    # Gzip compression
+    gzip on;
+    gzip_vary on;
+    gzip_proxied any;
+    gzip_comp_level 6;
+    gzip_types text/plain text/css text/xml application/json application/javascript application/rss+xml application/atom+xml image/svg+xml;
+
     # Static assets with caching (proxy to Flask)
     location /pos/static/ {
         proxy_pass http://nexus_pos;
@@ -63,7 +87,7 @@ server {
     }
 }
 
-# ─── Dashboard (admin subdomain) ───
+# ─── Dashboard admin (alternative access) ───
 server {
     listen 80;
     server_name admin.nexusautoparts.com.mx;