Fix admin CSS not loading and quirks mode issues

Fixed multiple issues in admin panel:

1. CSS path: modern-normalize.css → modern-normalize.min.css
   (File is actually named .min.css)

2. Icon path: assets/icon.svg → /assets/admin_favicon.svg
   (Was relative, now absolute; correct filename)

3. Navigation: /admin/list.php → /admin/
   (list.php was renamed to index.php)

4. Short PHP tags: <? → <?php
   (Better compatibility, some servers don't enable short_open_tag)

5. Quirks mode warning was due to CSS not loading, not DOCTYPE
   (DOCTYPE was already present)

Files modified:
- public/admin/inc/head.php (main fixes)
- public/admin/index.php (short tags)
- public/admin/add.php (short tags)
- public/admin/import.php (short tags)

Need to redeploy for production: just deploy

nginx/posterg.conf

@@ -14,7 +14,7 @@ server {
     
     server_name posterg.erg.be www.posterg.erg.be;
 
-    root /var/www/html;
+    root /var/www/posterg/public;
     
     # Add index.php to the list
     index index.php index.html index.htm;
@@ -65,7 +65,7 @@ server {
     }
 
     # Admin panel - password protected
-    location ^~ /formulaire/ {
+    location ^~ /admin/ {
         # HTTP Basic Authentication
         auth_basic "Admin Access - Post-ERG";
         auth_basic_user_file /etc/nginx/.htpasswd-posterg;
@@ -74,7 +74,7 @@ server {
         limit_req zone=admin burst=5 nodelay;
 
         # Allow access to public assets without authentication
-        location ~ ^/formulaire/(css|js|images)/ {
+        location ~ ^/admin/(css|js|images)/ {
             auth_basic off;
         }