feat: reseller self-service portal (clients + licenses + tickets)

- ResellerPortalView with 3 tabs:
  1. Kunden: list agency's clients + create new client (POST /api/aos/clients)
  2. Lizenzen: list agency-filtered licenses + create new license with
     live pricing preview (base + agency discount + yearly discount = net)
  3. Tickets: agency-scoped ticket inbox + escalate button
- Top: agency selector (pick which approved reseller to act as)
- Full integration: store, sidebar, top-bar, command-menu, i18n, app-shell
- Mobile responsive, German labels, no indigo/blue

src/store/aos.ts

@@ -14,6 +14,7 @@ export type ViewId =
   | 'support'
   | 'licenses'
   | 'agency'
+  | 'reseller_portal'
   | 'instances'
   | 'contracts'
   | 'expenses'