feat(fleet): Phase B — fleet overview UI + GET /api/fleet read endpoint

Tenant-scoped fleet read: GET /api/fleet returns agent_hosts (host
metrics) each with their game_instances, plus a summary
(host/instance/online counts). FleetView lists host cards (status, CPU/
mem/disk/uptime/last-heartbeat) with their instances (game, state badge,
uptime); honest empty state -> Server page when no hosts. New 'Fleet'
sidebar nav item across all four game profiles, /fleet route. Store
follows the no-throw-on-fetch pattern (error state, never bricks). The
marketing hero made real from the live fleet tables.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -343,6 +343,12 @@ const panelRoutes: RouteRecordRaw[] = [
         component: () => import('@/views/admin/AlertsView.vue'),
         meta: { title: 'Alerts — Corrosion' },
       },
+      {
+        path: 'fleet',
+        name: 'fleet',
+        component: () => import('@/views/admin/FleetView.vue'),
+        meta: { title: 'Fleet — Corrosion', requiresAuth: true },
+      },
       // Platform Admin views (super-admin only)
       {
         path: 'admin',