feat(widgets): scale chart axes/tooltips with unit + scale options

Consistent graph scaling across every line-chart widget. A new shared
frontend/src/lib/metricFormat.ts picks a decimal prefix (kB/MB/GB, kbps/Mbps,
Gbps, …) from the series magnitude and formats values; LineSeriesChart accepts
unit + scale and formats both the Y-axis ticks and the tooltip with the SAME
prefix (one consistent unit per axis). MetricChartWidget (Prometheus) and
QbittorrentSpeedWidget pass the widget config through; qBit speed defaults to
bytes/sec → MB/s.

WidgetConfigDialog now renders `enum` schema fields as a <Select> dropdown, so
the backend's unit/scale Literal enums become consistent pickers in every graph
widget's config (and any future enum option).

Decimal (x1000) prefixes by default (matches Mbps/MB/s/Grafana).

Tests: 13 new metricFormat tests (auto/fixed scaling, percent, seconds,
nulls, trailing-zero trimming). 179/179 frontend tests pass; tsc + ESLint clean.

frontend/src/widgets/MetricChartWidget.tsx

@@ -2,6 +2,7 @@ import { Alert, AlertDescription } from "@/components/ui/alert";
 import { Skeleton } from "@/components/ui/skeleton";
 import { LineSeriesChart } from "../components/LineSeriesChart";
 import type { ChartSeries } from "../components/LineSeriesChart";
+import type { MetricScale, MetricUnit } from "../lib/metricFormat";
 import { SectionCard } from "../components/SectionCard";
 import { useWidgetData } from "../hooks/useWidgets";
 import type { WidgetInstance } from "../types";
@@ -29,7 +30,11 @@ export function MetricChartWidget({
 					<AlertDescription>{data.error}</AlertDescription>
 				</Alert>
 			) : series && series.length > 0 ? (
-				<LineSeriesChart series={series} />
+				<LineSeriesChart
+					series={series}
+					unit={widget.config.unit as MetricUnit}
+					scale={widget.config.scale as MetricScale}
+				/>
 			) : (
 				<Alert>
 					<AlertDescription>