feat(networks): add network detail page with paginated host list and contextual back button

- API: add get_network(id) server function
- NetworkDetailPage at /networks/:id — network name + CIDR header, paginated
  host table (Name, IP, Ports, Apps) linking to /hosts/:id?back=/networks/:id
- Networks list: make network name a link to its detail page
- HostDetailPage: read ?back= query param to show "← Network" or "← Hosts"
  and navigate to the correct destination

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

src/client/host_detail.rs

@@ -6,7 +6,7 @@
 //   - Delete button  : opens a confirmation modal, then navigates back to /hosts
 
 use leptos::prelude::*;
-use leptos_router::hooks::{use_navigate, use_params_map};
+use leptos_router::hooks::{use_navigate, use_params_map, use_query_map};
 
 use crate::api::{
     hosts::{AddHostPort, DeleteHost, RemoveHostPort, UpdateHost, get_host_detail},
@@ -67,6 +67,18 @@ pub fn HostDetailPage() -> impl IntoView {
             .unwrap_or(0)
     };
 
+    // Optional `?back=<url>` query parameter — used when arriving from a network
+    // detail page so the back button returns there instead of the hosts list.
+    let query = use_query_map();
+    let back_url = move || {
+        query.read().get("back")
+            .map(|s| s.to_string())
+            .unwrap_or_else(|| "/hosts".to_string())
+    };
+    let back_label = move || {
+        if back_url().starts_with("/networks/") { "← Network" } else { "← Hosts" }
+    };
+
     let update_action      = ServerAction::<UpdateHost>::new();
     let add_port_action    = ServerAction::<AddHostPort>::new();
     let remove_port_action = ServerAction::<RemoveHostPort>::new();
@@ -169,7 +181,9 @@ pub fn HostDetailPage() -> impl IntoView {
                         view! {
                             // ── Page header ──────────────────────────────────
                             <div class="page-header detail-page-header">
-                                <a class="back-btn" href="/hosts">"← Hosts"</a>
+                                <a class="back-btn" href=move || back_url()>
+                                    {move || back_label()}
+                                </a>
                                 <h1 class="detail-page-title">{move || name_sig.get()}</h1>
                             </div>