refactor: move host-specific settings to hosts directory

- Move defaultNameServers from network/dns.nix to hosts/servers/default.nix and hosts/workstations/default.nix
- Move SMTP configuration from network/smtp.nix to hosts/servers/default.nix and hosts/workstations/default.nix
- Move proxy configuration from network/proxy.nix to hosts/servers/default.nix and hosts/workstations/default.nix
- Move NTP servers from network/time.nix to hosts/default.nix
- Remove network/proxy.nix and network/smtp.nix (host-specific settings don't belong in network/)
- Update network/default.nix to only import dns.nix and time.nix

This refactoring separates environment-specific network parameters (domain, timezone)
from host-specific settings (DNS servers, SMTP, proxy, NTP servers), making the
configuration more logical and maintainable.

environments/sample/network/dns.nix

@@ -4,9 +4,6 @@
   # Sample DNS configuration for the sample environment
   domain = "sample.lagraula.fr";
 
-  # Default DNS servers for this environment
-  defaultNameServers = [ "10.40.128.10" ];
-
   # Forwarders for this environment
   forwarders = [ "1.1.1.1" "8.8.8.8" ];
 
@@ -15,7 +12,4 @@
 
   # Recursion policy
   recursion = "AllowOnlyForPrivateNetworks";
-
-  # Email for Let's Encrypt
-  letsEncryptEmail = "xavier@lagraula.fr";
-}
+}