fix(gauge): render single value arc with correct Tailwind v4 colors

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.

frontend/src/components/LineSeriesChart.tsx

@@ -72,10 +72,10 @@ export function LineSeriesChart({
 				<Tooltip
 					labelFormatter={(label) => formatTime(Number(label))}
 					contentStyle={{
-						backgroundColor: "hsl(var(--popover))",
-						border: "1px solid hsl(var(--border))",
+						backgroundColor: "var(--color-popover)",
+						border: "1px solid var(--color-border)",
 						borderRadius: "0.5rem",
-						color: "hsl(var(--popover-foreground))",
+						color: "var(--color-popover-foreground)",
 					}}
 				/>
 				{series.map((s, i) => (