feat(grafana-metric-gateway): slice 2 — rename widgets to Metric*

git mv PrometheusChartWidget→MetricChartWidget, PrometheusGaugeWidget→
MetricGaugeWidget, PrometheusMeanWidget→MetricMeanWidget (+ 3 test files,
R100 history preserved). Update registry imports/refs + barrel exports.
Adapt PrometheusMetricWidget for §3.4 Option A: read normalized {result:
[{label,points}]} series shape (last-point extraction) instead of old Prom
{resultType,result} vector. PrometheusMetricWidget NOT renamed (design §3.1).
GM-111/112/116 satisfied. All gates: 151 vitest, build exit 0, lint 0 errors.

frontend/src/widgets/index.ts

@@ -1,8 +1,8 @@
 export { AlertmanagerAlertsWidget } from "./AlertmanagerAlertsWidget";
 export { BackupsWidget } from "./BackupsWidget";
-export { PrometheusChartWidget } from "./PrometheusChartWidget";
-export { PrometheusGaugeWidget } from "./PrometheusGaugeWidget";
-export { PrometheusMeanWidget } from "./PrometheusMeanWidget";
+export { MetricChartWidget } from "./MetricChartWidget";
+export { MetricGaugeWidget } from "./MetricGaugeWidget";
+export { MetricMeanWidget } from "./MetricMeanWidget";
 export { JellyfinWidget } from "./JellyfinWidget";
 export { JellyfinNowPlayingWidget } from "./JellyfinNowPlayingWidget";
 export { PrometheusMetricWidget } from "./PrometheusMetricWidget";