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/run/RunSetup.tsx

@@ -691,7 +691,7 @@ function SessionPicker({
                         s.status === "active"
                           ? "bg-status-success/10 text-status-success border-status-success/30"
                           : s.status === "completed"
-                            ? "bg-sky-500/10 text-sky-300 border-sky-500/30"
+                            ? "bg-sky-500/10 text-sky-700 dark:text-sky-300 border-sky-500/30"
                             : s.status === "error"
                               ? "bg-status-danger/10 text-status-danger border-status-danger/30"
                               : "bg-surface-3 text-fg-secondary border-border"