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/components/aos/top-bar.tsx

@@ -29,6 +29,7 @@ const TITLES: Record<ViewId, { title: string; crumb: string }> = {
   support: { title: 'Support', crumb: 'Lieferung / Tickets' },
   licenses: { title: 'Lizenzen', crumb: 'Betrieb / Lizenzen' },
   agency: { title: 'Reseller', crumb: 'Betrieb / Reseller' },
+  reseller_portal: { title: 'Reseller-Portal', crumb: 'Betrieb / Reseller-Portal' },
   instances: { title: 'Instanzen', crumb: 'Betrieb / Instanzen' },
   contracts: { title: 'Verträge', crumb: 'Betrieb / Verträge' },
   expenses: { title: 'Ausgaben', crumb: 'Betrieb / Ausgaben' },