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/workflows/ErrorPropagationMap.tsx

@@ -92,7 +92,7 @@ export function ErrorPropagationMap({ data }: ErrorPropagationMapProps) {
             height="24"
             viewBox="0 0 24 24"
             fill="none"
-            stroke="#10b981"
+            stroke="rgb(var(--status-success))"
             strokeWidth="2"
             strokeLinecap="round"
             strokeLinejoin="round"
@@ -174,7 +174,7 @@ export function ErrorPropagationMap({ data }: ErrorPropagationMapProps) {
                     </div>
                     <span
                       className="text-xs font-semibold tabular-nums w-7 text-right transition-colors"
-                      style={{ color: isHovered ? color : "#9ca3af" }}
+                      style={{ color: isHovered ? color : "rgb(var(--fg-secondary))" }}
                     >
                       {d.count}
                     </span>