refactor: move sleep to capture hub, remove from main nav

Changes:
1. Added sleep entry to CaptureHub (between Activity and Guide)
   - Icon: 🌙
   - Label: "Schlaf"
   - Sub: "Schlafdaten erfassen oder Apple Health importieren"
   - Color: #7B68EE (purple)
   - Route: /sleep

2. Removed sleep from main bottom navigation
   - Nav link removed (was 6 items → now 5 items)
   - Moon icon import removed (no longer used)
   - Route /sleep remains active (Widget + CaptureHub links work)

3. Widget link unchanged
   - SleepWidget.jsx still links to /sleep ✓
   - Dashboard → Widget → /sleep works

Result:
- Consistent UX: All data entry under "Erfassen"
- Clean navigation: 5 main nav items (was 6)
- Sleep accessible via: Dashboard Widget or Erfassen → Schlaf

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -1,6 +1,6 @@
 import { useEffect } from 'react'
 import { BrowserRouter, Routes, Route, NavLink, useNavigate } from 'react-router-dom'
-import { LayoutDashboard, PlusSquare, TrendingUp, BarChart2, Settings, LogOut, Moon } from 'lucide-react'
+import { LayoutDashboard, PlusSquare, TrendingUp, BarChart2, Settings, LogOut } from 'lucide-react'
 import { ProfileProvider, useProfile } from './context/ProfileContext'
 import { AuthProvider, useAuth }       from './context/AuthContext'
 import { setProfileId }                from './utils/api'
@@ -38,7 +38,6 @@ function Nav() {
     { to:'/',         icon:<LayoutDashboard size={20}/>, label:'Übersicht' },
     { to:'/capture',  icon:<PlusSquare size={20}/>,      label:'Erfassen'  },
     { to:'/history',  icon:<TrendingUp size={20}/>,      label:'Verlauf'   },
-    { to:'/sleep',    icon:<Moon size={20}/>,            label:'Schlaf'    },
     { to:'/analysis', icon:<BarChart2 size={20}/>,       label:'Analyse'   },
     { to:'/settings', icon:<Settings size={20}/>,        label:'Einst.'    },
   ]

frontend/src/pages/CaptureHub.jsx

@@ -45,6 +45,13 @@ const ENTRIES = [
     to: '/activity',
     color: '#D4537E',
   },
+  {
+    icon: '🌙',
+    label: 'Schlaf',
+    sub: 'Schlafdaten erfassen oder Apple Health importieren',
+    to: '/sleep',
+    color: '#7B68EE',
+  },
   {
     icon: '📖',
     label: 'Messanleitung',