Rebase services-as-hub-ia onto mobile-responsive-parity

Combine both branches into a single coherent branch:
- Full mobile responsive parity (useIsMobile, MobileCardRow, SheetForm,
  .mobile-touch-target, mobile cards, SheetForm forms, 44px targets,
  dirty-state confirm, TablePagination, refetchIntervalInBackground).
- Full services-as-hub IA (data-driven nav, service-page tab skeleton,
  new service types, Authentik directory + messaging, named dashboards,
  legacy routes 404, Observability split, Jellyseerr absorbed).

Enhancement: service tabs now use mobile-parity primitives:
- MediaTab: MobileCardRow below md (title/size/HDR/library/year) +
  TablePagination; DataTable at md+ (desktop branch preserved).
- FilesTab: MobileCardRow below md (name/type/size/modified) +
  handleRowClick; DataTable at md+.
- ServicePage: SheetForm branch below md (open-on-mount, sticky header
  + save bar, cancel navigates back to /services, dirty-state guard).
- Dashboard: single-column + section anchors below md (from mobile-parity)
  + empty-state CTA (from services-hub).
- App.tsx: useIsMobile() replaces inline matchMedia (from mobile-parity)
  + data-driven useNavItems (from services-hub).
- Backup tables (BackupAlerts/Jobs/Runs) already have MobileCardRow from
  mobile-parity; JobsTab inherits mobile behavior through its sub-components.

Conflict resolutions:
- Backend: entirely from services-hub (mobile didn't touch it).
- Deleted pages (Media/FileBrowser/Actions/Users/UsersPage/Applications/
  ObservabilityPage/BackupsPage + hooks/useUsers + tests): kept deleted
  (services-hub deleted them; content moved into service tabs).
- New service-tabs/*: from services-hub, enhanced with mobile patterns.
- App.tsx: services-hub's data-driven nav + mobile-parity's useIsMobile.
- Dashboard.tsx: merged (services-hub CTA + mobile-parity sections/anchors).
- ServicePage.tsx: services-hub's tab skeleton + mobile-parity's SheetForm.
- Primitives (useIsMobile/mobile-card/sheet-form/etc.): from mobile-parity.

117 frontend tests pass (mobile-parity's 122 - 5 deleted page tests +
services-hub's new tab/dashboard tests); 271 backend tests pass; lint/
build green both sides.

backend/src/media_library_viewer_api/integrations/authentik.py

@@ -0,0 +1,35 @@
+"""Authentik service definition.
+
+Authentik is the user-directory source (replacing the Jellyfin-backed Users
+page). Its directory API is queried via :class:`AuthentikClient` and surfaced
+on the Authentik service page (Users + Messaging tabs). OIDC authentication
+is unchanged -- this service type is for the directory, not SSO.
+"""
+
+from __future__ import annotations
+
+from media_library_viewer_api.integrations.base import (
+    SecretField,
+    ServiceBaseUrl,
+    ServiceConfigBase,
+    ServiceDefinition,
+)
+
+
+class AuthentikConfig(ServiceConfigBase):
+    """Non-secret Authentik connection config."""
+
+    base_url: ServiceBaseUrl
+    timeout_seconds: int = 10
+
+
+DEFINITION = ServiceDefinition(
+    service_type="authentik",
+    name="Authentik",
+    description="User directory and identity provider integration.",
+    config_model=AuthentikConfig,
+    secret_fields=[
+        SecretField(key="api_token", label="API token", required=True),
+    ],
+    widget_kinds=[],
+)