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/lanes/FolderBrowseModal.tsx

@@ -99,7 +99,7 @@ export function FolderBrowseModal({
               className="flex w-full items-center gap-2 rounded px-2 py-1.5 text-left text-xs text-fg-primary hover:bg-surface-2"
             >
               {entry.isGitRepo ? (
-                <FolderGit2 className="h-3.5 w-3.5 text-blue-400" />
+                <FolderGit2 className="h-3.5 w-3.5 text-blue-700 dark:text-blue-400" />
               ) : (
                 <FolderOpen className="h-3.5 w-3.5 text-fg-muted" />
               )}