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.
The gauge rendered as multiple black rings. Two causes:
1. recharts RadialBarChart draws each data entry as a CONCENTRIC RING, not an
arc segment, so the 3 "track band" entries + value produced 4 nested rings.
Render a single value arc over a neutral background track instead, colored
by status, with the readout absolutely centered (replacing the -mt-12 hack).
2. The fills used hsl(var(--primary)) / hsl(var(--chart-1)) etc., but this
project's Tailwind v4 theme (index.css) defines colors as --color-* holding
full hex values (--color-primary: #4f8cff). So the references were doubly
invalid (wrong name + hsl() wrapping a hex) -> invalid SVG fill defaults to
black. Use var(--color-*) directly, with the semantically correct chart
colors: ok=--color-chart-2 (green), warn=--color-chart-3 (amber),
crit=--color-chart-4 (red).
Also fix the same hsl(var(--x)) -> var(--color-x) bug in LineSeriesChart's
tooltip contentStyle (popover/border/popover-foreground). The line stroke
palette already used the correct var(--color-chart-N) form.
166/166 frontend tests pass; typecheck + ESLint clean.
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