M5-T06: add Energy device management UI and sidebar entry

frontend/src/components/AppSidebar.tsx

@@ -1,18 +1,16 @@
 /**
  * AppSidebar — vertical sidebar navigation for all protected pages.
  *
- * Nav items: Home / Records / Config
+ * Nav items: Home / Records / Energy / Config
  * Utilities: ColorSchemeToggle + LogoutButton (at bottom)
  *
  * Current route is highlighted via useLocation().
  * Mobile: burger button toggles the navbar open/closed (handled by AppShell context).
- *
- * NOTE: Energy nav item is intentionally absent — added in M5-T06 once /energy exists.
  */
 
 import { NavLink, Stack, Divider, Tooltip, ActionIcon, useMantineColorScheme, useComputedColorScheme, AppShell, Text, Group } from '@mantine/core'
 import { Link, useLocation, useNavigate } from 'react-router-dom'
-import { Home, List, Settings, Sun, Moon, LogOut } from 'react-feather'
+import { Home, List, Settings, Sun, Moon, LogOut, Zap } from 'react-feather'
 import { useQueryClient } from '@tanstack/react-query'
 import apiClient from '../api/client'
 
@@ -40,6 +38,12 @@ const NAV_ENTRIES: NavEntry[] = [
     icon: <List size={18} />,
     testId: 'nav-records',
   },
+  {
+    to: '/energy',
+    label: 'Energy',
+    icon: <Zap size={18} />,
+    testId: 'nav-energy',
+  },
   {
     to: '/config',
     label: 'Config',