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/EventDetail.tsx

@@ -379,7 +379,7 @@ function ScalarValue({ value }: { value: unknown }) {
   if (value == null) return <span className="text-fg-muted italic">null</span>;
   if (typeof value === "boolean") {
     const color = value
-      ? "text-green-400 border-green-500/30 bg-green-500/10"
+      ? "text-green-700 dark:text-green-400 border-green-500/30 bg-green-500/10"
       : "text-fg-secondary border-border-light/30 bg-surface-4/10";
     return (
       <span className={`inline-block px-2 py-0.5 rounded border ${color}`}>{String(value)}</span>