feat(11-02): add Sites to sidebar navigation and tenant index page

- MapPin icon and Sites nav link in sidebar Fleet section
- Tenant index shows Sites count card in 3-column grid
- "Manage sites" link added to tenant index bottom links

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -14,6 +14,7 @@ import {
   ClipboardList,
   Wifi,
   BarChart3,
+  MapPin,
 } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { useAuth, isSuperAdmin, isTenantAdmin } from '@/lib/auth'
@@ -110,6 +111,15 @@ export function Sidebar() {
               },
             ]
           : []),
+        ...(!isSuperAdmin(user) && user?.tenant_id
+          ? [
+              {
+                label: 'Sites',
+                href: `/tenants/${user.tenant_id}/sites`,
+                icon: MapPin,
+              },
+            ]
+          : []),
         {
           label: 'Wireless',
           href: '/wireless',