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.

frontend/src/integrations/registry.test.ts

@@ -23,6 +23,7 @@ describe("service registry", () => {
 	it("binds widget kinds per service", () => {
 		expect(SERVICE_REGISTRY.grafana.widgets.map((w) => w.kind)).toEqual([
 			"link",
+			"panel",
 		]);
 		expect(SERVICE_REGISTRY.alertmanager.widgets.map((w) => w.kind)).toEqual([
 			"active_alerts",