feat: add centralized environment configuration

- Add dns.nix: environment-specific DNS domain, name servers, forwarders
- Add time.nix: NTP servers (gateway), timezone, hardware clock
- Add proxy.nix: placeholder for future proxy settings (no proxy for now)
- Add smtp.nix: SMTP relay configuration for system emails
- Add default.nix: unified entry point for all network parameters

These files allow environment-specific settings to be managed in one place
and imported by host configurations.

nixos-infra/environments/dev/network/time.nix

@@ -0,0 +1,16 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # NTP servers for this environment
+  # The gateway acts as the NTP server (IPv4 .1, IPv6 ::1)
+  ntpServers = [
+    "10.10.128.1"    # IPv4 gateway
+    "fd00::1"        # IPv6 gateway
+  ];
+
+  # Time zone for this environment
+  timeZone = "Europe/Paris";
+
+  # Hardware clock setting
+  hardwareClock = "UTC";
+}