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/ImportHistory.tsx

@@ -625,7 +625,7 @@ export function ImportHistory() {
               <ResultStat
                 label={t("import.result.backfilled", { count: result.backfilled ?? 0 })}
                 value={result.backfilled ?? 0}
-                color="text-blue-400"
+                color="text-blue-700 dark:text-blue-400"
               />
               <ResultStat
                 label={t("import.result.skipped", { count: result.skipped })}
@@ -672,12 +672,12 @@ export function ImportHistory() {
               <ResultStat
                 label={t("import.backupResult.events", { count: backupResult.events })}
                 value={backupResult.events}
-                color="text-violet-300"
+                color="text-violet-700 dark:text-violet-300"
               />
               <ResultStat
                 label={t("import.backupResult.pricing", { count: backupResult.model_pricing })}
                 value={backupResult.model_pricing}
-                color="text-cyan-300"
+                color="text-cyan-700 dark:text-cyan-300"
               />
             </div>
             <p className="text-[11px] text-fg-muted">
@@ -737,7 +737,7 @@ function ModeButton({
     >
       <div
         className={`flex items-center gap-1.5 text-xs font-medium mb-1 ${
-          active ? "text-blue-400" : "text-fg-secondary"
+          active ? "text-blue-700 dark:text-blue-400" : "text-fg-secondary"
         }`}
       >
         {icon}