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/app/(admin)/development/companion/page.tsx

@@ -1,39 +0,0 @@
-'use client'
-
-import { PagePurpose } from '@/components/common/PagePurpose'
-import { getModuleByHref } from '@/lib/navigation'
-import { GraduationCap, Construction } from 'lucide-react'
-
-export default function CompanionPage() {
-  const moduleInfo = getModuleByHref('/development/companion')
-
-  return (
-    <div className="space-y-6">
-      {moduleInfo && (
-        <PagePurpose
-          title={moduleInfo.module.name}
-          purpose={moduleInfo.module.purpose}
-          audience={moduleInfo.module.audience}
-          collapsible={true}
-          defaultCollapsed={true}
-        />
-      )}
-
-      <div className="bg-white border border-slate-200 rounded-xl p-8 text-center">
-        <div className="flex justify-center mb-4">
-          <div className="p-4 bg-slate-100 rounded-full">
-            <GraduationCap className="w-12 h-12 text-slate-400" />
-          </div>
-        </div>
-        <h2 className="text-xl font-semibold text-slate-800 mb-2">Companion Dev</h2>
-        <p className="text-slate-600 mb-4">
-          Lesson-Modus Entwicklung fuer strukturiertes Lernen.
-        </p>
-        <div className="inline-flex items-center gap-2 px-4 py-2 bg-amber-50 border border-amber-200 rounded-lg text-amber-700">
-          <Construction className="w-4 h-4" />
-          <span className="text-sm font-medium">In Entwicklung</span>
-        </div>
-      </div>
-    </div>
-  )
-}