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/WorkflowStats.tsx

@@ -71,7 +71,7 @@ function formatDurationSec(sec: number): string {
 
 function successRateColor(rate: number): string {
   if (rate > 90) return "text-status-success";
-  if (rate > 70) return "text-yellow-400";
+  if (rate > 70) return "text-yellow-700 dark:text-yellow-400";
   return "text-status-danger";
 }
 
@@ -325,7 +325,7 @@ export function WorkflowStats({ stats }: WorkflowStatsProps) {
         label={t("stats.avgAgentDepth")}
         value={stats.avgDepth.toFixed(1)}
         icon={GitFork}
-        accentClass="text-indigo-400"
+        accentClass="text-indigo-700 dark:text-indigo-400"
         calculationKey="stats.tooltip.calc.depth"
         interp={interpAvgDepth(stats.avgDepth)}
         metricPhraseKey="stats.tooltip.phrase.depth"
@@ -352,7 +352,7 @@ export function WorkflowStats({ stats }: WorkflowStatsProps) {
         label={t("stats.mostCommonFlow")}
         value={topFlowLabel}
         icon={ArrowRightLeft}
-        accentClass="text-violet-400"
+        accentClass="text-violet-700 dark:text-violet-400"
         calculationKey="stats.tooltip.calc.topFlow"
         interp={interpTopFlow(topFlow?.source ?? null, topFlow?.target ?? null)}
         metricPhraseKey="stats.tooltip.phrase.topFlow"
@@ -361,7 +361,7 @@ export function WorkflowStats({ stats }: WorkflowStatsProps) {
         label={t("stats.avgCompactions")}
         value={stats.avgCompactions.toFixed(1)}
         icon={Layers}
-        accentClass="text-cyan-400"
+        accentClass="text-cyan-700 dark:text-cyan-400"
         calculationKey="stats.tooltip.calc.compactions"
         interp={interpAvgCompactions(stats.avgCompactions)}
         metricPhraseKey="stats.tooltip.phrase.compactions"