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/src/media_library_viewer_api/routers/widgets.py

@@ -103,10 +103,18 @@ def list_builtin_kinds() -> list[BuiltinWidgetKindInfo]:
 
 @router.get("/instances")
 def list_instances(
+    service_id: str | None = None,
+    scope: str | None = None,
     store: SettingsStore = Depends(get_settings_store),
 ) -> list[dict[str, Any]]:
-    """Return all persisted widget instances."""
-    return [WidgetInstance(**widget).model_dump() for widget in store.list_widgets()]
+    """Return widget instances, optionally filtered.
+
+    - ``?service_id=X``: only widgets for service X.
+    - ``?scope=dashboard``: only widgets with NULL service_id.
+    - ``?scope=service``: only widgets with a non-null service_id.
+    - No params: all widgets (backward-compatible).
+    """
+    return [WidgetInstance(**widget).model_dump() for widget in store.list_widgets(service_id=service_id, scope=scope)]
 
 
 @router.post("/instances", status_code=status.HTTP_201_CREATED)