RewriteEngine On

# Redirect admin directory root to login
RewriteRule ^$ login.php [L,R=301]

# Allow direct access to specific files
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .* - [L]

# Redirect everything else to login
RewriteRule .* login.php [L,R=301]