Add Jellyfin Now Playing + Grafana Panel embed widgets

Two new additive widget kinds:

Jellyfin 'now_playing': like the existing 'activity' widget but filters
to only sessions with active playback (NowPlayingItem present + not
paused). Shows who's actually watching right now. The 'activity' kind
is unchanged (shows all sessions including idle).

Grafana 'panel': embeds a single Grafana panel directly in the app via
an iframe, using Grafana's /d-solo/ endpoint (renders one panel without
dashboard chrome, kiosk=tv). Configurable dashboard_uid, panel_id, and
time range (from/to, defaults now-1h/now). Includes a fallback 'Open in
Grafana' link for when embedding is blocked by X-Frame-Options/CSP. The
'link' kind is unchanged (still builds a deep-link URL).

Backend: new widget configs + definitions on jellyfin/grafana; source
adapter logic (session filter for now_playing; d-solo embed URL for
panel); 6 new tests.

Frontend: JellyfinNowPlayingWidget + GrafanaPanelWidget components;
registry bindings; 6 new tests.

278 backend tests pass (+6); 127 frontend tests pass (+6); lint/build
green both sides.

backend/src/media_library_viewer_api/widgets/sources.py

@@ -97,7 +97,7 @@ class StaticWidgetSource:
 
 
 class GrafanaWidgetSource:
-    """Build a Grafana deep-link (no embedding)."""
+    """Build a Grafana deep-link or panel embed URL."""
 
     async def fetch(self, service: ServiceRecord | None, widget_kind: str, config: dict[str, Any]) -> dict[str, Any]:
         try:
@@ -107,6 +107,19 @@ class GrafanaWidgetSource:
             dashboard_uid = config.get("dashboard_uid")
             if not dashboard_uid:
                 return {"error": "dashboard_uid is required"}
+
+            if widget_kind == "panel":
+                panel_id = config.get("panel_id")
+                if panel_id is None:
+                    return {"error": "panel_id is required"}
+                from_ts = config.get("from_ts", "now-1h")
+                to_ts = config.get("to_ts", "now")
+                embed_url = (
+                    f"{base_url}/d-solo/{dashboard_uid}/manage?panelId={panel_id}&from={from_ts}&to={to_ts}&kiosk=tv"
+                )
+                return {"embed_url": embed_url}
+
+            # Default: deep-link
             url = f"{base_url}/d/{dashboard_uid}"
             panel_id = config.get("panel_id")
             if panel_id is not None:
@@ -208,6 +221,10 @@ class JellyfinWidgetSource:
                 asyncio.to_thread(client.sessions),
                 timeout=timeout,
             )
+            if widget_kind == "now_playing":
+                sessions = [
+                    s for s in sessions if s.get("NowPlayingItem") and not s.get("PlayState", {}).get("IsPaused", True)
+                ]
             rows = _map_sessions_to_activity_rows(sessions)
             return {"sessions": rows}
         except asyncio.TimeoutError: