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/hosts/workstations/default.nix

@@ -2,13 +2,13 @@
 
 let
   # Import environment-specific network parameters
-  env = import ../../../network { };
+  env = import ../../../../network { };
 in {
-  # Common settings for all workstations in the sample environment
+  # Common settings for all workstations in the production environment
   # This file is imported by all workstation configurations
 
   # Default DNS servers for this environment
-  networking.nameServers = [ "10.40.128.10" ];
+  networking.nameServers = [ "10.40.128.10" "10.40.128.11" ];
 
   # SMTP relay configuration
   services.postfix = {
@@ -16,7 +16,7 @@ in {
     relayHost = "smtp.lagraula.fr";
     relayPort = 587;
     useTLS = true;
-    fromAddress = "noreply@sample.lagraula.fr";
+    fromAddress = "noreply@prod.lagraula.fr";
   };
 
   # Proxy configuration for workstations (none for now)
@@ -32,4 +32,4 @@ in {
   # - Workstation-specific services
   # - Workstation-specific security policies
   # - Workstation-specific monitoring/alerting
-}
+}