Add "View Networks": front-facing subnet browse + detail with attached devices

Subnets previously lived only under Settings, easily confused with the Network
Devices asset list. Add a front-facing browse + detail:

- Nav: rename "Network" -> "Network Devices"; add "View Networks" (subnets), both
  under Assets (network plugin get_navigation_items; frontend fallback matched).
- /networks (SubnetsBrowse): all subnets with name / CIDR / type / VLAN / notes,
  searchable, row-click to detail.
- /networks/:id (SubnetDetail): the subnet (CIDR, network address, type, VLAN,
  gateway, notes) plus the network devices whose primary IP falls inside its
  CIDR - get_subnet now computes that membership in Python (a device's IP lives
  in a Communication row, so it is not a plain SQL join).

Verified on the import DB: 37 networks list (real WJ subnets), detail renders.

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

frontend/src/views/AppLayout.vue

@@ -156,7 +156,8 @@ const defaultNav = [
   { name: 'Map', icon: 'map', route: '/map', position: 4 },
   { name: 'Machines', icon: 'cog', route: '/machines', position: 10 },
   { name: 'PCs', icon: 'desktop', route: '/pcs', position: 15 },
-  { name: 'Network', icon: 'network-wired', route: '/network', position: 18 },
+  { name: 'Network Devices', icon: 'network-wired', route: '/network', position: 18 },
+  { name: 'View Networks', icon: 'globe', route: '/networks', position: 19 },
   { name: 'Printers', icon: 'printer', route: '/printers', position: 20 },
   { name: 'USB Devices', icon: 'usb', route: '/usb', position: 45 },
   { name: 'Applications', icon: 'app-window', route: '/applications', position: 30, section: 'information' },