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/conversation/ToolCallBlock.tsx

@@ -156,7 +156,7 @@ function renderInput(toolUse: TranscriptContent) {
     return (
       <div className="space-y-2">
         <div className="flex items-center gap-1.5 text-xs text-fg-secondary">
-          <FileText className="w-3.5 h-3.5 text-violet-400" />
+          <FileText className="w-3.5 h-3.5 text-violet-700 dark:text-violet-400" />
           <span className="font-mono">{obj.file_path}</span>
           {obj.replace_all === true && (
             <span className="text-[10px] uppercase tracking-wider text-status-warning/80 bg-status-warning/10 border border-status-warning/20 rounded px-1.5 py-0.5">
@@ -178,7 +178,7 @@ function renderInput(toolUse: TranscriptContent) {
   if (tool === "read" && typeof obj.file_path === "string") {
     return (
       <div className="flex items-center gap-1.5 text-xs text-fg-secondary bg-surface-4/40 border border-surface-3 rounded-md px-3 py-2">
-        <FileText className="w-3.5 h-3.5 text-sky-400 flex-shrink-0" />
+        <FileText className="w-3.5 h-3.5 text-sky-700 dark:text-sky-400 flex-shrink-0" />
         <span className="font-mono break-all">{obj.file_path}</span>
         {(typeof obj.offset === "number" || typeof obj.limit === "number") && (
           <span className="text-fg-muted font-mono ml-auto flex-shrink-0">
@@ -198,7 +198,7 @@ function renderInput(toolUse: TranscriptContent) {
           <span className="text-fg-muted font-mono uppercase tracking-wider text-[10px]">
             Pattern
           </span>
-          <code className="font-mono text-cyan-300 bg-surface-4 border border-surface-3 rounded px-1.5 py-0.5">
+          <code className="font-mono text-cyan-700 dark:text-cyan-300 bg-surface-4 border border-surface-3 rounded px-1.5 py-0.5">
             {obj.pattern}
           </code>
         </div>