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.
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