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/Sessions.tsx

@@ -436,7 +436,7 @@ export function Sessions() {
                           </p>
                           {session.source && session.source !== "local" && (
                             <span
-                              className="inline-flex items-center gap-1 text-[10px] font-semibold text-sky-300 bg-sky-500/10 border border-sky-500/25 px-1.5 py-0.5 rounded-full"
+                              className="inline-flex items-center gap-1 text-[10px] font-semibold text-sky-700 dark:text-sky-300 bg-sky-500/10 border border-sky-500/25 px-1.5 py-0.5 rounded-full"
                               title={t("remoteSourceBadgeTitle", "Collected from a remote machine")}
                             >
                               <Server className="w-2.5 h-2.5" />