Service IA refinement: nav naming, instance tabs, config to Settings, configurable Overview

Four coupled changes to the services-as-hub IA:

1. Nav entries use service TYPE names (Jellyfin, SSH Tasks, Alertmanager,
   Grafana, Prometheus, Backups, Authentik) instead of conceptual names
   (Media, Files, Actions, Alerts, Users). ssh_tasks collapses to one
   entry ('SSH Tasks') instead of two. The content tabs inside each
   service page surface the concepts (Files, Actions).

2. Service page gains a two-level tab structure when multiple enabled
   instances of the same type exist: instance tabs on top ([Main Jellyfin]
   [Backup Jellyfin]), content tabs below ([Overview] [Media] [Requests]
   [Widgets]). Clicking an instance tab navigates to the sibling's route.
   Single instance: no instance tabs. Replaces the dropdown switcher.

3. Config tab (connection fields, secrets, enable/disable, delete) moves
   from the service page to Settings > Services tab. The service page
   becomes a PURE operational view (Overview + content tabs + Widgets) --
   no save/delete/config state. Settings gains a 4th tab 'Services' with
   ServiceConfigEditor per instance (schema-driven config fields, secrets
   with leave-blank-to-keep semantics, ConfirmDialog on delete).

4. Overview tab is now a configurable widget grid per service instance.
   Each instance manages its own set of widgets on its Overview. Backend
   widget list endpoints gain ?service_id= and ?scope= (dashboard|service)
   filter params; the main Dashboard uses scope=dashboard to exclude
   service-scoped widgets. The OverviewTab reuses WidgetInstanceCard +
   WidgetConfigDialog. Empty state CTA for instances with no widgets.

All service-tab stubs are replaced; stubs.tsx deleted.

272 backend tests pass (+1 widget filter); 121 frontend tests pass (+3
instance-tabs + OverviewTab); lint/build green both sides.

backend/tests/test_widgets.py

@@ -90,6 +90,44 @@ def test_create_backups_widget(client):
     assert response.status_code == 201
 
 
+def test_widget_filtering_by_service_id_and_scope(client):
+    """Test ?service_id= and ?scope= query params on GET /api/widgets/instances."""
+    service = _make_grafana_service(client)
+    # Create a dashboard-scoped (built-in) widget + a service-scoped widget.
+    client.post(
+        "/api/widgets/instances",
+        json={"widget_kind": "static", "title": "Note", "config": {"text": "hi"}},
+    )
+    client.post(
+        "/api/widgets/instances",
+        json={
+            "service_id": service["id"],
+            "widget_kind": "link",
+            "title": "Dash",
+            "config": {"dashboard_uid": "o"},
+        },
+    )
+
+    # No filter: both widgets.
+    all_widgets = client.get("/api/widgets/instances").json()
+    assert len(all_widgets) == 2
+
+    # Filter by service_id: only the service-scoped one.
+    by_service = client.get(f"/api/widgets/instances?service_id={service['id']}").json()
+    assert len(by_service) == 1
+    assert by_service[0]["service_id"] == service["id"]
+
+    # scope=dashboard: only the built-in (NULL service_id).
+    dashboard_scope = client.get("/api/widgets/instances?scope=dashboard").json()
+    assert len(dashboard_scope) == 1
+    assert dashboard_scope[0]["service_id"] is None
+
+    # scope=service: only the non-null service_id widget.
+    service_scope = client.get("/api/widgets/instances?scope=service").json()
+    assert len(service_scope) == 1
+    assert service_scope[0]["service_id"] == service["id"]
+
+
 def test_unknown_builtin_kind_rejected(client):
     response = client.post(
         "/api/widgets/instances",