Network: consolidate into one tabbed hub; subnet devices span all asset types

Replaces the two flat "Network Devices" + "View Networks" nav entries with a
single "Network" entry opening a tabbed hub: Devices | Networks | VLANs
(NetworkHub renders the existing device list, the subnet browse, and the VLAN
list; VLANs is now reachable outside Settings). /network -> hub; /networks
redirects to the Networks tab; subnet detail stays at /networks/:id.

Subnet "Devices on this network" now matches ANY asset whose primary IP falls in
the CIDR (PCs, printers, machines, measuring tools - not just network devices),
computed on the core Communication + Asset tables; each row links to its typed
detail (extension id resolved lazily/guarded per plugin). Fixes the empty list -
printers and PCs carry IPs and now appear (e.g. 35 devices on 10.80.92.0/24).

Also: subnet-browse search uses the standard form-control styling; dropped the
redundant per-tab page header.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/network/SubnetsBrowse.vue

@@ -1,11 +1,7 @@
 <template>
   <div>
-    <div class="page-header">
-      <h1>Networks</h1>
-    </div>
-
     <div class="filters">
-      <input v-model="search" type="text" placeholder="Search networks..." class="search-input" />
+      <input v-model="search" type="text" placeholder="Search networks..." class="form-control" />
     </div>
 
     <div class="card">