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/pages/__tests__/Dashboard.heatmap.test.ts

@@ -0,0 +1,30 @@
+/**
+ * @file Dashboard.heatmap.test.ts
+ * @description Pins the activity heatmap's colour ramp to the theme tokens: it
+ * runs from the page surface to the accent in every theme, instead of a fixed
+ * near-black indigo that vanished into dark and blotted out light themes.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect } from "vitest";
+import { cellColor } from "../Dashboard";
+
+describe("heatmap cellColor", () => {
+  it("paints an empty day as the bare surface", () => {
+    expect(cellColor(0, 10)).toBe("rgb(var(--surface-0))");
+  });
+
+  it("mixes the accent into the surface, more for busier days, and never a fixed rgb", () => {
+    const low = cellColor(1, 100);
+    const high = cellColor(100, 100);
+    for (const c of [low, high]) {
+      expect(c).toMatch(
+        /^color-mix\(in srgb, rgb\(var\(--accent\)\) \d+%, rgb\(var\(--surface-0\)\)\)$/
+      );
+    }
+    const pct = (c: string) => Number(/(\d+)%/.exec(c)?.[1]);
+    expect(pct(high)).toBe(100);
+    expect(pct(low)).toBeGreaterThan(0);
+    expect(pct(low)).toBeLessThan(pct(high));
+  });
+});