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/widgets/index.ts

@@ -1,6 +1,7 @@
 export { AlertmanagerAlertsWidget } from "./AlertmanagerAlertsWidget";
 export { BackupsWidget } from "./BackupsWidget";
 export { GrafanaLinkWidget } from "./GrafanaLinkWidget";
+export { GrafanaChartWidget } from "./GrafanaChartWidget";
 export { JellyfinWidget } from "./JellyfinWidget";
 export { PrometheusMetricWidget } from "./PrometheusMetricWidget";
 export { SshTaskWidget } from "./SshTaskWidget";