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/ActivityFeed.tsx

@@ -341,7 +341,9 @@ export function ActivityFeed() {
             <p className="text-xs text-fg-muted">
               {t("subtitle")}
               {paused && (
-                <span className="ml-2 text-yellow-400">{t("paused", { count: bufferCount })}</span>
+                <span className="ml-2 text-yellow-700 dark:text-yellow-400">
+                  {t("paused", { count: bufferCount })}
+                </span>
               )}
             </p>
           </div>