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/builtins/contacts/plugin.py

@@ -13,6 +13,7 @@ from app.plugins.manifest import (
     FrontendDashboardWidget,
     FrontendMenuItem,
     FrontendPageRoute,
+    MiniAppContribution,
     PluginManifest,
     PluginRouteDef,
 )
@@ -60,6 +61,26 @@ class ContactsPlugin(BasePlugin):
         ],
         events=[],
         migrations=[],
+        miniapps=[
+            MiniAppContribution(
+                app_id="contacts_stats",
+                name="Kontakt-Zähler",
+                icon="Building2",
+                description="Firmen- und Kontakt-Zähler (persönliche StatCards).",
+                permission="contacts:read",
+                settings_schema={
+                    "fields": [
+                        {"name": "show_companies", "label": "Firmen anzeigen", "type": "boolean", "default": True},
+                        {"name": "show_persons", "label": "Personen anzeigen", "type": "boolean", "default": True},
+                    ]
+                },
+                col_span=2,
+                row_span=1,
+                hosts=["chat", "dashboard", "window"],
+                component="@/components/dashboard/ContactsStatsWidget",
+                order=5,
+            ),
+        ],
         dashboard_widgets=[
             FrontendDashboardWidget(
                 id="recent_contacts",