feat(admin): Admin-Panel-Frontend (/admin)

- content/admin.md (appshell, noindex), renderAdmin in hosting-app.js:
  Kennzahlen-Kacheln (Kunden/Abos/Instanzen/MRR) + Kundentabelle
- Admin-Link im Konto-Header (nur wenn account.is_admin)
- Admin-Styles (Stat-Kacheln, Tabelle) in custom.css

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

content/admin.md

@@ -0,0 +1,7 @@
+---
+title: Admin
+layout: appshell
+hostingpage: admin
+toc: false
+noindex: true
+---