RewriteEngine On

# Laisse passer les fichiers/répertoires existants
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]

# Route SPA: / et /s/<id> -> index.php
RewriteRule ^(s/[^/]+)?$ index.php [L,QSA]
