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/core/system_miniapps.py

@@ -0,0 +1,63 @@
+"""Core-owned system MiniApps (Phase M4).
+
+Host-level MiniApps that are not owned by a single plugin: audit activity
+feed and system metrics. They register in the universal registry with
+``plugin_name="system"`` at app startup and unregister with the registry
+reset (tests) — they never depend on plugin activation state.
+
+Permissions follow the owning data source:
+- audit_activity -> audit:read (audit log route guard, CORE_PERMISSIONS)
+- system_metrics  -> settings:read (Roadmap M4; the /system/dashboard
+  endpoint itself stays require_admin — the widget degrades gracefully
+  with a permission hint for non-admins)
+"""
+
+from __future__ import annotations
+
+from app.plugins.miniapp_registry import get_miniapp_registry
+
+SYSTEM_PLUGIN_NAME = "system"
+
+
+def register_system_miniapps() -> None:
+    """Register the core system MiniApps in the universal registry."""
+    registry = get_miniapp_registry()
+
+    registry.register(
+        app_id="audit_activity",
+        name="Aktivitäten",
+        icon="History",
+        description="Letzte Aktivitäten aus dem Audit-Log (Benutzer, Aktion, Zeitpunkt).",
+        plugin_name=SYSTEM_PLUGIN_NAME,
+        permission="audit:read",
+        settings_schema={
+            "fields": [
+                {
+                    "name": "max_items",
+                    "label": "Max. Einträge",
+                    "type": "number",
+                    "default": 10,
+                }
+            ]
+        },
+        col_span=2,
+        row_span=1,
+        hosts=["chat", "dashboard", "window"],
+        component="@/components/dashboard/AuditActivityWidget",
+        order=40,
+    )
+
+    registry.register(
+        app_id="system_metrics",
+        name="System Status",
+        icon="Server",
+        description="Datenbank-, Redis-, Worker- und API-Metriken (Administration).",
+        plugin_name=SYSTEM_PLUGIN_NAME,
+        permission="settings:read",
+        settings_schema={},
+        col_span=2,
+        row_span=1,
+        hosts=["chat", "dashboard", "window"],
+        component="@/components/dashboard/SystemMetricsWidget",
+        order=50,
+    )