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/integrations/grafana.py

@@ -26,6 +26,15 @@ class GrafanaLinkWidgetConfig(WidgetConfigBase):
     panel_id: int | None = None
 
 
+class GrafanaPanelWidgetConfig(WidgetConfigBase):
+    """Embed a single Grafana panel via iframe."""
+
+    dashboard_uid: str
+    panel_id: int
+    from_ts: str = "now-1h"
+    to_ts: str = "now"
+
+
 DEFINITION = ServiceDefinition(
     service_type="grafana",
     name="Grafana",
@@ -43,5 +52,13 @@ DEFINITION = ServiceDefinition(
             default_config={"dashboard_uid": ""},
             refresh_interval_ms=0,
         ),
+        widget_kind(
+            kind="panel",
+            name="Panel embed",
+            description="Embed a Grafana panel directly.",
+            model_cls=GrafanaPanelWidgetConfig,
+            default_config={"dashboard_uid": "", "panel_id": 1, "from_ts": "now-1h", "to_ts": "now"},
+            refresh_interval_ms=0,
+        ),
     ],
 )