Align with AM stack SOPs: required files, nginx hardening, mobile CSS

- Add robots.txt, sitemap.xml (16 pages), 404.html, 500.html per SOP
- nginx: allow robots.txt/sitemap.xml explicitly, fix error_page to 404.html, deny _template.html, remove txt from deny list, fix API proxy comment
- Convert residential.png to residential.webp per image SOP
- components.css: mobile nav breakpoint 768→1023px, 360px ultra-narrow query, overflow-x:clip, inline grid collapse overrides
- blog/index.html: placeholder blog listing page with 3 article cards

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

nginx.conf

@@ -9,7 +9,7 @@ server {
         deny all;
         return 404;
     }
-    location ~* \.(env|env\.example|conf|yml|yaml|py|pyc|md|txt|sh|sql|log|bak|old|swp|dockerfile)$ {
+    location ~* \.(env|env\.example|conf|yml|yaml|py|pyc|md|sh|sql|log|bak|old|swp|dockerfile)$ {
         deny all;
         return 404;
     }
@@ -18,7 +18,15 @@ server {
         return 404;
     }
 
-    # API proxy — strip /api/ prefix, forward to Node.js service
+    location ~* /_template\.html$ {
+        deny all;
+        return 404;
+    }
+
+    location = /robots.txt  { access_log off; }
+    location = /sitemap.xml { access_log off; }
+
+    # API proxy — strip /api/ prefix, forward to Python API service
     location /api/ {
         proxy_pass         http://api:3001/;
         proxy_http_version 1.1;
@@ -51,5 +59,5 @@ server {
     gzip_types text/html text/css application/javascript image/svg+xml;
     gzip_min_length 1024;
 
-    error_page 404 /index.html;
+    error_page 404 /404.html;
 }