* fix: keep nginx IPv6 listen only when IPv6 is available (#4596)
Commit 2c48b1896 (1.45.2) made the internal nginx proxy listen on the IPv6
wildcard ('listen [::]:7575;'), which aborts nginx startup on hosts/VMs with
IPv6 disabled:
nginx: [emerg] socket() [::]:7575 failed (97: Address family not supported by protocol)
A plain revert would reintroduce #2697 and break IPv6-only/dual-stack
environments where this listener is required. Instead, only emit the IPv6
listen directive when the host actually has IPv6 configured.
Reuse the envsubst step run.sh already performs: compute NGINX_LISTEN_IPV6
from /proc/net/if_inet6 and substitute it into the template. proc files always
report size 0, so presence is detected by grepping the file (not 'test -s').
* fix: honor HOMARR_DISABLE_IPV6 override in nginx config generation
Review feedback on #6539: give users an explicit opt-out from the IPv6
listener even when the host reports IPv6. The issue reporter (#4596)
requested such an env var, and the pattern matches the existing
ENABLE_DNS_CACHING IPv6 toggle. Auto-detection behavior is unchanged when
the var is unset. Validated in containers: override wins, dual-stack
preserved by default, IPv4-only fallback on IPv6-less hosts.
Thomas Camlong
ajnart