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/main.py

@@ -218,6 +218,11 @@ async def lifespan(app: FastAPI):
     registry.initialize(get_migration_engine(), app)
     registry.discover_builtins()
 
+    # Core system MiniApps (Phase M4): host-level, independent of plugin state
+    from app.core.system_miniapps import register_system_miniapps
+
+    register_system_miniapps()
+
     # Install discovered builtin plugins and activate only those marked active in DB
     from sqlalchemy import select as sa_select
     from sqlalchemy.ext.asyncio import async_sessionmaker