feat(models): add domain structs and CIDR validation

Add shared models (Network, Host, Port, Application, ApplicationPort)
with serde derives for Leptos server function serialization.
Add server/validation.rs with valider_ip_dans_reseau() and 5 unit tests.
Gate SSR-only modules (config, validation) with #[cfg(feature = "ssr")].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -836,6 +836,15 @@ dependencies = [
  "rustversion",
 ]
 
+[[package]]
+name = "ipnetwork"
+version = "0.20.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf466541e9d546596ee94f9f69590f89473455f88372423e0008fc1a7daf100e"
+dependencies = [
+ "serde",
+]
+
 [[package]]
 name = "itertools"
 version = "0.14.0"
@@ -1597,10 +1606,12 @@ dependencies = [
  "axum",
  "console_error_panic_hook",
  "dotenvy",
+ "ipnetwork",
  "leptos",
  "leptos_axum",
  "leptos_meta",
  "leptos_router",
+ "serde",
  "thiserror 1.0.69",
  "tokio",
  "tower-http 0.5.2",