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/pages/Workflows.tsx

@@ -224,7 +224,7 @@ export function Workflows() {
       <div className="card p-4 space-y-3">
         <div>
           <h2 className="text-sm font-semibold text-fg-secondary flex items-center gap-2">
-            <Workflow className="w-4 h-4 text-violet-400" />
+            <Workflow className="w-4 h-4 text-violet-700 dark:text-violet-400" />
             {t("runs.title")}
           </h2>
           <p className="text-xs text-fg-muted mt-0.5">{t("runs.subtitle")}</p>