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/terminalTheme.ts

@@ -0,0 +1,94 @@
+/**
+ * @file terminalTheme.ts
+ * @description The xterm colour theme for lane terminals. The terminal is
+ * ALWAYS dark — Claude Code's TUI picks its own colours for a dark background,
+ * so a light terminal would make its text unreadable — and is painted with the
+ * palette of the DARK theme the user picked (`themeFor.dark`). Colours are read
+ * from the CSS variables in `index.css` on a hidden probe carrying
+ * `class="dark" data-theme=<id>`, so no hex is duplicated here.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+import type { ITheme } from "@xterm/xterm";
+
+/** Reads one CSS variable (an RGB triplet like `17 17 19`) by name. */
+export type VarReader = (name: string) => string;
+
+function toHex(triplet: string): string | undefined {
+  const parts = triplet
+    .trim()
+    .split(/[\s,]+/)
+    .map(Number);
+  if (parts.length !== 3 || parts.some((n) => !Number.isFinite(n))) return undefined;
+  return "#" + parts.map((n) => Math.round(n).toString(16).padStart(2, "0")).join("");
+}
+
+/** Mix a hex colour toward white by `amount` (0..1) — the ANSI "bright" twin. */
+function lighten(hex: string | undefined, amount: number): string | undefined {
+  if (!hex) return undefined;
+  const rgb = [1, 3, 5].map((i) => parseInt(hex.slice(i, i + 2), 16));
+  return (
+    "#" +
+    rgb
+      .map((c) =>
+        Math.round(c + (255 - c) * amount)
+          .toString(16)
+          .padStart(2, "0")
+      )
+      .join("")
+  );
+}
+
+/**
+ * Build an xterm theme from a theme's CSS variables. red/green/yellow come from
+ * `--status-*`, blue from `--accent`, black/white from the surface/fg ramp.
+ * Magenta and cyan stay the classic ANSI hues: no token keeps that hue in every
+ * theme (darcula's keyword is orange), and a program printing "magenta" should
+ * get magenta.
+ */
+export function buildTerminalTheme(read: VarReader): ITheme {
+  const v = (name: string) => toHex(read(name));
+  const red = v("--status-danger");
+  const green = v("--status-success");
+  const yellow = v("--status-warning");
+  const blue = v("--accent");
+  const magenta = "#c678dd";
+  const cyan = "#56b6c2";
+  return {
+    background: v("--surface-0"),
+    foreground: v("--fg-primary"),
+    cursor: v("--fg-primary"),
+    cursorAccent: v("--surface-0"),
+    selectionBackground: v("--border-light"),
+    black: v("--border-light"),
+    red,
+    green,
+    yellow,
+    blue,
+    magenta,
+    cyan,
+    white: v("--fg-secondary"),
+    brightBlack: v("--fg-muted"),
+    brightRed: lighten(red, 0.25),
+    brightGreen: lighten(green, 0.25),
+    brightYellow: lighten(yellow, 0.25),
+    brightBlue: lighten(blue, 0.25),
+    brightMagenta: lighten(magenta, 0.25),
+    brightCyan: lighten(cyan, 0.25),
+    brightWhite: v("--fg-primary"),
+  };
+}
+
+/** Resolve the dark theme `id`'s variables on a hidden probe and build the theme. */
+export function terminalThemeFor(id: string): ITheme {
+  const probe = document.createElement("div");
+  probe.className = "dark";
+  if (id !== "default") probe.dataset.theme = id;
+  probe.style.display = "none";
+  document.body.appendChild(probe);
+  try {
+    const style = getComputedStyle(probe);
+    return buildTerminalTheme((name) => style.getPropertyValue(name));
+  } finally {
+    probe.remove();
+  }
+}