feat(marketplace): UI fuer Plugin-Marketplace — Modul 5/16 des UI-Backlogs

ERSTES Modul ueber die Phase-Q-Plugin-Architektur: Registrierung komplett
ueber das Plugin-Manifest (page_routes + menu_items) — routes/index.tsx und
Sidebar.tsx wurden NICHT angefasst. Der Komponenten-Map-Generator wired den
lazy import automatisch (38 Komponenten).

Backend existierte vollstaendig (listings mit search/tags/pagination,
listing-detail, install mit Ed25519-Signatur-Verify, verify, categories;
marketplace:read/admin + require_admin fuer Install), Frontend hatte 0%
Abdeckung.

- api/marketplace.ts: TanStack-Hooks (useMarketplaceListings mit
  search/tags/pagination, useMarketplaceListing, useMarketplaceCategories,
  useInstallFromMarketplace, useVerifyMarketplacePlugin)
- pages/Marketplace.tsx: Suche, Tag-Filter-Chips, Listing-Karten (Name,
  Version, Author, Beschreibung, Tags, Download-Counter, Verified-Badge,
  Preis/Kostenlos), Install mit Confirm (Admin-only via is_system_admin),
  Signatur-Verify, Ergebnis-Banner, Pagination — No-Access-Card ohne
  marketplace:read
- Manifest: page_route /marketplace + menu_item (Store-Icon, order 85,
  permission marketplace:read)
- i18n marketplace.* + nav.marketplace de/en

Verifikation: Vitest 10/10 (Rendering, No-Access, Empty/Error, Karten,
  Search+Tags, Admin-Gating, Install-Flow mit+ohne Confirm, Verify-Flow) ·
  tsc exit 0 · production build exit 0 · Backend-Regressionen
  (route-order, m5-miniapps) 10/10 · compileall sauber · Manifest-Check:
  page_route + menu_item korrekt.

frontend/src/generated/pluginComponents.generated.ts

@@ -45,6 +45,7 @@ export const PLUGIN_COMPONENT_MAP: Record<string, LazyComponentFactory> = {
   '@/pages/ImportExport': () => import('@/pages/ImportExport').then((m) => ({ default: m.ImportExportPage })),
   '@/pages/Mail': () => import('@/pages/Mail').then((m) => ({ default: m.MailPage })),
   '@/pages/MailSettings': () => import('@/pages/MailSettings').then((m) => ({ default: m.MailSettingsPage })),
+  '@/pages/Marketplace': () => import('@/pages/Marketplace').then(normalizeModule),
   '@/pages/ProactiveAISettings': () => import('@/pages/ProactiveAISettings').then((m) => ({ default: m.ProactiveAISettings })),
   '@/pages/Reports': () => import('@/pages/Reports').then((m) => ({ default: m.ReportsPage })),
   '@/pages/SettingsGroups': () => import('@/pages/SettingsGroups').then((m) => ({ default: m.SettingsGroupsPage })),