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/workflows/OrchestrationDAG.tsx

@@ -587,7 +587,7 @@ export function OrchestrationDAG({ data, onNodeClick, selectedNode }: Orchestrat
       .attr("x", (_, i) => layerXPositions[i] ?? 0)
       .attr("y", 22)
       .attr("text-anchor", "middle")
-      .attr("fill", "#6b7280")
+      .attr("fill", "rgb(var(--fg-muted))")
       .attr("font-size", "10px")
       .attr("font-weight", "500")
       .attr("letter-spacing", "0.08em")
@@ -605,7 +605,7 @@ export function OrchestrationDAG({ data, onNodeClick, selectedNode }: Orchestrat
         .attr("y1", 32)
         .attr("x2", sepX)
         .attr("y2", svgHeight - PADDING_BOTTOM + 8)
-        .attr("stroke", "#1f1f30")
+        .attr("stroke", "rgb(var(--border))")
         .attr("stroke-width", 1)
         .attr("stroke-dasharray", "4 4");
     }
@@ -651,7 +651,7 @@ export function OrchestrationDAG({ data, onNodeClick, selectedNode }: Orchestrat
         .append("path")
         .attr("d", path)
         .attr("fill", "none")
-        .attr("stroke", isZero ? "#2a2a3d" : "#4f46e5")
+        .attr("stroke", isZero ? "rgb(var(--border-light))" : "#4f46e5")
         .attr("stroke-width", isZero ? 1 : stroke)
         .attr("stroke-opacity", isZero ? 0.3 : 0.55)
         .attr("stroke-linecap", "round");