feat(companion): Migrate Companion from admin-v2 to studio-v2 as pure lesson tool

Remove Companion module entirely from admin-v2. Rebuild in studio-v2 as a
focused lesson timer (no dashboard mode). Direct flow: start → active → ended.
Fix timer bug where lastTickRef reset prevented countdown. Add companion link
to Sidebar and i18n translations.

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

admin-v2/components/companion/companion-mode/StatsGrid.tsx

@@ -1,114 +0,0 @@
-'use client'
-
-import { Users, GraduationCap, BookOpen, FileCheck } from 'lucide-react'
-import { CompanionStats } from '@/lib/companion/types'
-
-interface StatsGridProps {
-  stats: CompanionStats
-  loading?: boolean
-}
-
-interface StatCardProps {
-  label: string
-  value: number
-  icon: React.ReactNode
-  color: string
-  loading?: boolean
-}
-
-function StatCard({ label, value, icon, color, loading }: StatCardProps) {
-  return (
-    <div className="bg-white border border-slate-200 rounded-xl p-4 hover:shadow-md transition-shadow">
-      <div className="flex items-start justify-between">
-        <div>
-          <p className="text-sm text-slate-500 mb-1">{label}</p>
-          {loading ? (
-            <div className="h-8 w-16 bg-slate-200 rounded animate-pulse" />
-          ) : (
-            <p className="text-2xl font-bold text-slate-900">{value}</p>
-          )}
-        </div>
-        <div className={`p-2 rounded-lg ${color}`}>
-          {icon}
-        </div>
-      </div>
-    </div>
-  )
-}
-
-export function StatsGrid({ stats, loading }: StatsGridProps) {
-  const statCards = [
-    {
-      label: 'Klassen',
-      value: stats.classesCount,
-      icon: <Users className="w-5 h-5 text-blue-600" />,
-      color: 'bg-blue-100',
-    },
-    {
-      label: 'Schueler',
-      value: stats.studentsCount,
-      icon: <GraduationCap className="w-5 h-5 text-green-600" />,
-      color: 'bg-green-100',
-    },
-    {
-      label: 'Lerneinheiten',
-      value: stats.learningUnitsCreated,
-      icon: <BookOpen className="w-5 h-5 text-purple-600" />,
-      color: 'bg-purple-100',
-    },
-    {
-      label: 'Noten',
-      value: stats.gradesEntered,
-      icon: <FileCheck className="w-5 h-5 text-amber-600" />,
-      color: 'bg-amber-100',
-    },
-  ]
-
-  return (
-    <div className="grid grid-cols-2 md:grid-cols-4 gap-4">
-      {statCards.map((card) => (
-        <StatCard
-          key={card.label}
-          label={card.label}
-          value={card.value}
-          icon={card.icon}
-          color={card.color}
-          loading={loading}
-        />
-      ))}
-    </div>
-  )
-}
-
-/**
- * Compact version of StatsGrid for sidebar or smaller spaces
- */
-export function StatsGridCompact({ stats, loading }: StatsGridProps) {
-  const items = [
-    { label: 'Klassen', value: stats.classesCount, icon: <Users className="w-4 h-4" /> },
-    { label: 'Schueler', value: stats.studentsCount, icon: <GraduationCap className="w-4 h-4" /> },
-    { label: 'Einheiten', value: stats.learningUnitsCreated, icon: <BookOpen className="w-4 h-4" /> },
-    { label: 'Noten', value: stats.gradesEntered, icon: <FileCheck className="w-4 h-4" /> },
-  ]
-
-  return (
-    <div className="bg-white border border-slate-200 rounded-xl p-4">
-      <h3 className="text-sm font-medium text-slate-500 mb-3">Statistiken</h3>
-      <div className="space-y-3">
-        {items.map((item) => (
-          <div key={item.label} className="flex items-center justify-between">
-            <div className="flex items-center gap-2 text-slate-600">
-              {item.icon}
-              <span className="text-sm">{item.label}</span>
-            </div>
-            {loading ? (
-              <div className="h-5 w-8 bg-slate-200 rounded animate-pulse" />
-            ) : (
-              <span className="font-semibold text-slate-900">{item.value}</span>
-            )}
-          </div>
-        ))}
-      </div>
-    </div>
-  )
-}