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/lib/__tests__/terminalTheme.test.ts

@@ -0,0 +1,71 @@
+/**
+ * @file terminalTheme.test.ts
+ * @description Tests for the xterm theme builder: CSS-variable RGB triplets
+ * become hex, status/accent map to the ANSI hues, bright twins are lighter,
+ * and the probe resolves a named dark theme's variables.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+import { describe, it, expect } from "vitest";
+import { buildTerminalTheme, terminalThemeFor } from "../terminalTheme";
+
+const vars: Record<string, string> = {
+  "--surface-0": " 17 17 19",
+  "--surface-3": "39 42 45",
+  "--fg-primary": "237 238 240",
+  "--fg-secondary": "176 180 186",
+  "--fg-muted": "105 110 119",
+  "--border-light": "67 72 78",
+  "--accent": "0 144 255",
+  "--status-success": "61 214 140",
+  "--status-danger": "255 149 146",
+  "--status-warning": "255 202 22",
+};
+
+describe("buildTerminalTheme", () => {
+  it("maps theme tokens onto the xterm palette", () => {
+    const t = buildTerminalTheme((n) => vars[n] ?? "");
+    expect(t.background).toBe("#111113");
+    expect(t.foreground).toBe("#edeef0");
+    expect(t.cursor).toBe("#edeef0");
+    expect(t.selectionBackground).toBe("#43484e");
+    expect(t.red).toBe("#ff9592");
+    expect(t.green).toBe("#3dd68c");
+    expect(t.yellow).toBe("#ffca16");
+    expect(t.blue).toBe("#0090ff");
+    expect(t.black).toBe("#43484e");
+    expect(t.brightBlack).toBe("#696e77");
+    expect(t.brightWhite).toBe("#edeef0");
+  });
+
+  it("makes each bright twin lighter than its base colour", () => {
+    const t = buildTerminalTheme((n) => vars[n] ?? "");
+    const sum = (hex: string) =>
+      [1, 3, 5].reduce((a, i) => a + parseInt(hex.slice(i, i + 2), 16), 0);
+    for (const c of ["red", "green", "yellow", "blue", "magenta", "cyan"] as const) {
+      const base = t[c]!;
+      const bright = t[`bright${c[0]!.toUpperCase()}${c.slice(1)}` as keyof typeof t] as string;
+      expect(sum(bright)).toBeGreaterThan(sum(base));
+    }
+  });
+
+  it("leaves a colour unset when its variable is missing, so xterm keeps its default", () => {
+    const t = buildTerminalTheme(() => "");
+    expect(t.background).toBeUndefined();
+    expect(t.red).toBeUndefined();
+  });
+});
+
+describe("terminalThemeFor", () => {
+  it("resolves variables on a dark probe scoped to the theme id, then removes it", () => {
+    const style = document.createElement("style");
+    style.textContent = `.dark { --surface-0: 1 2 3; } .dark[data-theme="darcula"] { --surface-0: 30 31 32; }`;
+    document.head.appendChild(style);
+    try {
+      expect(terminalThemeFor("default").background).toBe("#010203");
+      expect(terminalThemeFor("darcula").background).toBe("#1e1f20");
+      expect(document.body.children).toHaveLength(0);
+    } finally {
+      style.remove();
+    }
+  });
+});