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/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: frontend/src/components
 
 ## role
-Reusable UI component library providing presentation-layer building blocks (tables, cards, charts, dialogs, widgets) for the frontend application.
+Shared UI component library providing reusable React display, table, dialog, card, chart, and widget components for the frontend application.
 ## parent
 index: frontend/src/.pi-map.index.md
 map: frontend/src/.pi-map.md
@@ -28,6 +28,7 @@ map: frontend/src/.pi-map.md
 - PinnedServiceLink.tsx
 - SectionCard.tsx
 - SelectionRailCard.tsx
+- ServiceTestPanel.tsx
 - SessionActivityPanel.tsx
 - TabbedCard.tsx
 - WidgetConfigDialog.tsx