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/AgentCard.tsx

@@ -182,7 +182,9 @@ export function AgentCard({ agent, session, label, onClick }: AgentCardProps) {
         <div className="flex items-center gap-2.5 min-w-0 overflow-hidden">
           <div
             className={`w-7 h-7 rounded-md flex items-center justify-center flex-shrink-0 ${
-              isMain ? "bg-accent/15 text-accent" : "bg-violet-500/15 text-violet-400"
+              isMain
+                ? "bg-accent/15 text-accent"
+                : "bg-violet-500/15 text-violet-700 dark:text-violet-400"
             }`}
           >
             {isMain ? <Bot className="w-3.5 h-3.5" /> : <GitBranch className="w-3.5 h-3.5" />}