feat(ui): terminal, charts and low-contrast text follow the selected theme

- The lane terminal (xterm) takes its palette from the selected DARK theme —
  it stays dark in light mode, as a terminal should — and repaints live when
  the theme changes (`client/src/lib/terminalTheme.ts`).
- Charts on the Dashboard and Workflows pages draw their grid, axes, labels
  and tooltips from theme tokens instead of hardcoded near-black/white, and
  the activity heatmap ramps from the page surface to the accent, so it reads
  on every theme instead of going near-black on light ones.
- Status/brand text that only worked on dark backgrounds
  (`text-*-300/400`) gets a darker light-mode shade (`text-*-700
  dark:text-*-300`) across sessions, conversations, workflows, config and
  settings. Categorical chart colours are kept.

client/src/components/StatusBadge.tsx

@@ -91,7 +91,7 @@ function ReasonChip({ reason }: { reason: AwaitingReason }) {
       className={`inline-flex items-center gap-1 rounded-full border px-1.5 text-[10px] font-medium leading-4 ${
         cfg.urgent
           ? "bg-status-warning/15 border-status-warning/25 text-status-warning"
-          : "bg-yellow-500/10 border-yellow-500/20 text-yellow-400/90"
+          : "bg-yellow-500/10 border-yellow-500/20 text-yellow-700/90 dark:text-yellow-400/90"
       }`}
     >
       <Icon className="w-2.5 h-2.5 flex-shrink-0" aria-hidden="true" />