feat(M4): System-Rueckbau — Dashboard-Inhalte als MiniApps, Core = reiner Host (#362)

- system_miniapps.py: audit_activity (audit:read, settings max_items) + system_metrics (settings:read) als Core-Apps in Registry
- base.py-Fix: native Manifest-MiniApps reichen component durch (M1-Luecke)
- contacts-Manifest: contacts_stats (ContactsStatsWidget, contacts:read, show_companies/show_persons)
- Seed-Fix: nur renderbare Apps (component) landen im Dashboard-Layout
- Frontend: ContactsStatsWidget, AuditActivityWidget, SystemMetricsWidget; MiniAppHost-Registry +3
- Dashboard.tsx = reiner Host (26 Z.); Page-Tests auf Pure-Host umgeschrieben
- Tests: M4 7/7 (TDD rot->gruen), Backend-Regression 46/46, Vitest 22/22, tsc clean, build OK

app/plugins/base.py

@@ -116,6 +116,7 @@ class BasePlugin(ABC):
                 col_span=getattr(m, "col_span", 1),
                 row_span=getattr(m, "row_span", 1),
                 hosts=getattr(m, "hosts", None),
+                component=getattr(m, "component", ""),
                 order=getattr(m, "order", 100),
             )