Replace Grafana iframe panel with server-side chart widget

The iframe-based 'panel' widget didn't work: the browser couldn't
authenticate against the OIDC-protected Grafana (Authentik), and
iframes can't carry Bearer tokens or share cross-origin session
cookies. Result: blank iframe or login redirect.

Replace it with a 'chart' widget that queries Grafana's datasource
API server-side:

Backend (GrafanaWidgetSource): POSTs to /api/ds/query with the stored
api_key (which bypasses OIDC), using the widget's configured PromQL
query, datasource_uid, time range, and resolution. Normalizes Grafana's
frame-based response into a simple {series: [{label, points: [{t, v}]}]}
shape. The api_key is never exposed to the browser.

Frontend (GrafanaChartWidget): renders the series data as a recharts
LineChart with dark-mode-aware colors (Tailwind --chart-* tokens),
responsive container, custom tooltip, and per-series lines. Loading
skeleton, error Alert, and empty state. recharts ^3.9.2 added.

The 'link' widget kind (deep-link URL) is unchanged. The 'panel' kind
and GrafanaPanelWidget are fully removed.

Backend: 279 tests pass (+1 net: -2 panel + 3 chart). Frontend: 127
tests pass (net 0: -3 panel + 3 chart). Lint/build green both sides.

frontend/src/integrations/registry.ts

@@ -2,7 +2,7 @@ import type { ComponentType } from "react";
 import { AlertmanagerAlertsWidget } from "../widgets/AlertmanagerAlertsWidget";
 import { BackupsWidget } from "../widgets/BackupsWidget";
 import { GrafanaLinkWidget } from "../widgets/GrafanaLinkWidget";
-import { GrafanaPanelWidget } from "../widgets/GrafanaPanelWidget";
+import { GrafanaChartWidget } from "../widgets/GrafanaChartWidget";
 import { JellyfinWidget } from "../widgets/JellyfinWidget";
 import { JellyfinNowPlayingWidget } from "../widgets/JellyfinNowPlayingWidget";
 import { PrometheusMetricWidget } from "../widgets/PrometheusMetricWidget";
@@ -89,27 +89,37 @@ export const SERVICE_REGISTRY: Record<string, ServiceBinding> = {
 				component: GrafanaLinkWidget,
 			},
 			{
-				kind: "panel",
-				name: "Panel embed",
-				description: "Embed a Grafana panel directly.",
-				refreshIntervalMs: 0,
+				kind: "chart",
+				name: "Chart",
+				description: "Live time-series chart from a Grafana datasource query.",
+				refreshIntervalMs: 60_000,
 				defaultConfig: {
-					dashboard_uid: "",
-					panel_id: 1,
+					datasource_uid: "prometheus",
+					query: "",
 					from_ts: "now-1h",
 					to_ts: "now",
+					interval_ms: 30_000,
+					max_data_points: 100,
 				},
 				configSchema: {
 					type: "object",
 					properties: {
-						dashboard_uid: { type: "string" },
-						panel_id: { type: "integer" },
+						datasource_uid: {
+							type: "string",
+							description: "Grafana datasource UID (e.g. 'prometheus')",
+						},
+						query: {
+							type: "string",
+							description: "Query expression (e.g. PromQL)",
+						},
 						from_ts: { type: "string" },
 						to_ts: { type: "string" },
+						interval_ms: { type: "integer" },
+						max_data_points: { type: "integer" },
 					},
-					required: ["dashboard_uid", "panel_id"],
+					required: ["query"],
 				},
-				component: GrafanaPanelWidget,
+				component: GrafanaChartWidget,
 			},
 		],
 	},