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/clients/authentik.py

@@ -0,0 +1,115 @@
+"""Authentik directory API client.
+
+Authentik is the user-directory source (replacing the Jellyfin-backed Users
+page). This client wraps the Authentik REST API for browsing the user directory
+with pagination and search. OIDC authentication is unchanged — this client is
+for the directory, not SSO.
+"""
+
+from __future__ import annotations
+
+import logging
+from typing import Any
+
+import requests
+
+logger = logging.getLogger(__name__)
+
+
+class AuthentikClient:
+    """Small wrapper around the Authentik core directory API."""
+
+    def __init__(self, base_url: str, api_token: str, timeout: float = 10.0):
+        if not base_url:
+            raise ValueError("Authentik base_url is required")
+        if not api_token:
+            raise ValueError("Authentik API token is required")
+
+        self.base_url = base_url.rstrip("/")
+        if self.base_url.endswith("/api/v3"):
+            self.base_url = self.base_url[:-7]
+        self.api_token = api_token
+        self.timeout = timeout
+        self.session = requests.Session()
+        self.session.headers.update(
+            {
+                "Authorization": f"Bearer {api_token}",
+                "Accept": "application/json",
+            }
+        )
+
+    def get(self, path: str, **params: Any) -> Any:
+        """GET an Authentik endpoint and include useful response text on errors."""
+        clean_params = {k: v for k, v in params.items() if v is not None and v != ""}
+        logger.debug("Authentik GET %s params=%s", path, sorted(clean_params.keys()))
+        response = self.session.get(
+            f"{self.base_url}/api/v3{path}",
+            params=clean_params,
+            timeout=self.timeout,
+        )
+        try:
+            response.raise_for_status()
+        except requests.HTTPError as exc:
+            detail = response.text[:500]
+            logger.warning(
+                "Authentik GET %s failed status=%s url=%s",
+                path,
+                response.status_code,
+                response.url,
+            )
+            raise requests.HTTPError(
+                f"{response.status_code} for {response.url}: {detail}",
+                response=response,
+            ) from exc
+        logger.debug("Authentik GET %s ok status=%s", path, response.status_code)
+        return response.json()
+
+    def users(
+        self,
+        search: str | None = None,
+        page: int = 1,
+        page_size: int = 50,
+    ) -> dict[str, Any]:
+        """Return a normalized page of Authentik users.
+
+        Calls ``GET /api/v3/core/users/`` and normalizes the paginated
+        Authentik response into ``{items, total, page, page_size}``. Each item
+        is the raw Authentik user dict (pk, username, name, email, avatar, …)
+        so the frontend can pick the fields it needs.
+        """
+        payload = self.get(
+            "/core/users/",
+            search=search,
+            page=page,
+            page_size=page_size,
+        )
+        if not isinstance(payload, dict):
+            logger.warning("Authentik users payload was not a dict: %s", type(payload).__name__)
+            return {"items": [], "total": 0, "page": page, "page_size": page_size}
+
+        results = payload.get("results")
+        items: list[dict[str, Any]] = (
+            [item for item in results if isinstance(item, dict)] if isinstance(results, list) else []
+        )
+
+        pagination = payload.get("pagination") or {}
+        total = 0
+        if isinstance(pagination, dict):
+            try:
+                total = int(pagination.get("count") or 0)
+            except (TypeError, ValueError):
+                total = 0
+
+        logger.info(
+            "Authentik users page=%s page_size=%s -> %s items (total=%s)",
+            page,
+            page_size,
+            len(items),
+            total,
+        )
+        return {
+            "items": items,
+            "total": total,
+            "page": page,
+            "page_size": page_size,
+        }