Add fallback default in Caddyfile for DOMAIN_SUFFIX

Without a fallback, an unset DOMAIN_SUFFIX expands to an empty string,
making Caddy parse the site block as a second global options block and
fail to start. Using {:localhost} defaults to localhost.

proxy/Caddyfile

@@ -20,7 +20,7 @@
 }
 
 # HIY dashboard — served at your root domain.
-{$DOMAIN_SUFFIX} {
+{$DOMAIN_SUFFIX:localhost} {
     reverse_proxy server:3000
 }