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/widgets/JellyfinNowPlayingWidget.tsx

@@ -0,0 +1,41 @@
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import { Skeleton } from "@/components/ui/skeleton";
+import { SessionActivityPanel } from "../components/SessionActivityPanel";
+import { SectionCard } from "../components/SectionCard";
+import { useWidgetData } from "../hooks/useWidgets";
+import type { NowPlayingSession, WidgetInstance } from "../types";
+
+interface Props {
+	widget: WidgetInstance;
+	refreshIntervalMs: number;
+	description?: string;
+}
+
+export function JellyfinNowPlayingWidget({
+	widget,
+	refreshIntervalMs,
+	description,
+}: Props) {
+	const { data, isLoading } = useWidgetData(widget.id, refreshIntervalMs);
+	const sessions = data?.data?.sessions as NowPlayingSession[] | undefined;
+
+	return (
+		<SectionCard title={widget.title} description={description}>
+			{isLoading && !data ? (
+				<div className="flex flex-col gap-2">
+					<Skeleton className="h-4 w-3/4" />
+					<Skeleton className="h-4 w-1/2" />
+				</div>
+			) : data?.error ? (
+				<Alert variant="destructive">
+					<AlertDescription>{data.error}</AlertDescription>
+				</Alert>
+			) : Array.isArray(sessions) ? (
+				<SessionActivityPanel
+					sessions={sessions}
+					emptyMessage="No one is playing right now."
+				/>
+			) : null}
+		</SectionCard>
+	);
+}