feat(ui): Appearance settings — dark/light/system mode and named themes; fix Add lane silently doing nothing (#42)

- Settings → Appearance: mode Dark / Light / System (follows the OS live) and
  a named theme remembered per mode — dark: Default, Darcula, Apple Dark;
  light: Default, Apple Light, IntelliJ Light, Canvas. Each theme is a
  CSS-variable override block keyed by `data-theme` on <html>; the sidebar
  quick toggle gains System and swaps between the two remembered themes.
- An inline boot script in index.html applies the stored mode and theme
  before first paint, so a reload no longer flashes the default colours.
- Diff +/- rows are easier to tell apart on every theme: a /20 status tint,
  a 3px status bar and a bold coloured marker; the changed-span emphasis no
  longer vanishes on light themes.
- #42: the Add-lane modal (Repo mode / "Current branch") asks
  POST /api/lanes/ensure with `exact: true`. Without it a folder nested inside
  an existing lane resolved to that parent lane with created:false, so the
  modal closed and no new card appeared. When the folder really already has a
  lane, the modal now says which one instead of closing.

Closes #42

client/src/components/settings/AppearanceSettings.tsx

@@ -0,0 +1,128 @@
+/**
+ * @file AppearanceSettings.tsx
+ * @description Settings → Appearance: the Dark / Light / System mode switch and
+ * one named-theme picker per mode (state in `hooks/useTheme.ts`, palettes in
+ * `index.css`). Each card's swatch re-scopes the theme's CSS variables on its
+ * own element (`dark` class or `data-mode`, plus `data-theme`), so the preview
+ * is painted by the same tokens as the app — no hex is duplicated here.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { useTranslation } from "react-i18next";
+import { Monitor, Moon, Palette, Sun } from "lucide-react";
+import { THEMES, useTheme, type Theme, type ThemeMode } from "../../hooks/useTheme";
+
+const THEME_LABELS: Record<string, string> = {
+  darcula: "Darcula",
+  "apple-dark": "Apple Dark",
+  "apple-light": "Apple Light",
+  "intellij-light": "IntelliJ Light",
+  canvas: "Canvas",
+};
+
+const MODES: { mode: ThemeMode; Icon: typeof Moon }[] = [
+  { mode: "dark", Icon: Moon },
+  { mode: "light", Icon: Sun },
+  { mode: "system", Icon: Monitor },
+];
+
+function Swatch({ mode, name }: { mode: Theme; name: string }) {
+  return (
+    <div
+      aria-hidden
+      className={`${mode === "dark" ? "dark" : ""} rounded-md border border-border bg-surface-0 p-2 text-fg-primary`}
+      data-mode={mode}
+      data-theme={name === "default" ? undefined : name}
+    >
+      <div className="rounded bg-surface-3 p-2 shadow-[var(--shadow-card)] space-y-1.5">
+        <div className="flex items-center gap-1.5">
+          <span className="h-2 w-2 rounded-full bg-accent" />
+          <span className="h-1.5 w-10 rounded-full bg-fg-primary/70" />
+        </div>
+        <div className="font-mono text-[10px] leading-tight">
+          <span className="text-syntax-keyword">const</span>{" "}
+          <span className="text-syntax-function">x</span>{" "}
+          <span className="text-syntax-operator">=</span>{" "}
+          <span className="text-syntax-string">"ok"</span>
+        </div>
+        <div className="flex gap-1">
+          <span className="h-1.5 flex-1 rounded-full bg-status-success/40" />
+          <span className="h-1.5 flex-1 rounded-full bg-status-danger/40" />
+        </div>
+      </div>
+    </div>
+  );
+}
+
+export function AppearanceSettings() {
+  const { t } = useTranslation("settings");
+  const { mode, theme, themeFor, setMode, setThemeFor } = useTheme();
+
+  return (
+    <section id="appearance" className="scroll-mt-24">
+      <h3 className="text-sm font-medium text-fg-secondary flex items-center gap-2 mb-1">
+        <Palette className="w-4 h-4 text-fg-muted" />
+        {t("appearance.title")}
+      </h3>
+      <p className="text-xs text-fg-muted mb-4">{t("appearance.description")}</p>
+
+      <div className="card p-5 space-y-5">
+        <div>
+          <p className="text-xs font-medium text-fg-secondary mb-2">{t("appearance.mode")}</p>
+          <div className="inline-flex rounded-lg border border-border bg-surface-2 p-0.5">
+            {MODES.map(({ mode: m, Icon }) => (
+              <button
+                key={m}
+                type="button"
+                onClick={() => setMode(m)}
+                aria-pressed={mode === m}
+                className={`flex items-center gap-1.5 rounded-md px-3 py-1.5 text-xs font-medium transition-colors ${
+                  mode === m
+                    ? "bg-accent/20 text-accent"
+                    : "text-fg-secondary hover:bg-surface-3 hover:text-fg-primary"
+                }`}
+              >
+                <Icon className="w-3.5 h-3.5" />
+                {t(`appearance.modes.${m}`)}
+              </button>
+            ))}
+          </div>
+        </div>
+
+        {(["dark", "light"] as const).map((group) => (
+          <div key={group}>
+            <p className="text-xs font-medium text-fg-secondary mb-2">
+              {t(`appearance.${group}Themes`)}
+              {theme === group && (
+                <span className="ml-2 text-fg-muted font-normal">{t("appearance.active")}</span>
+              )}
+            </p>
+            <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 gap-3">
+              {THEMES[group].map((name) => {
+                const selected = themeFor[group] === name;
+                const label = THEME_LABELS[name] ?? t("appearance.default");
+                return (
+                  <button
+                    key={name}
+                    type="button"
+                    onClick={() => setThemeFor(group, name)}
+                    aria-pressed={selected}
+                    aria-label={`${t(`appearance.${group}Themes`)}: ${label}`}
+                    className={`rounded-lg border p-1.5 text-left transition-colors ${
+                      selected
+                        ? "border-accent ring-2 ring-accent/40"
+                        : "border-border hover:border-border-light"
+                    }`}
+                  >
+                    <Swatch mode={group} name={name} />
+                    <span className="block px-1 pt-1.5 text-xs text-fg-primary">{label}</span>
+                  </button>
+                );
+              })}
+            </div>
+          </div>
+        ))}
+      </div>
+    </section>
+  );
+}

client/src/components/settings/__tests__/AppearanceSettings.test.tsx

@@ -0,0 +1,60 @@
+/**
+ * @file AppearanceSettings.test.tsx
+ * @description Tests for the Settings → Appearance section: the mode switch and the per-mode theme cards write through useTheme to `<html>`.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { AppearanceSettings } from "../AppearanceSettings";
+
+const root = document.documentElement;
+
+describe("AppearanceSettings", () => {
+  beforeEach(() => {
+    localStorage.clear();
+    root.classList.remove("dark");
+    root.removeAttribute("data-theme");
+    root.removeAttribute("data-mode");
+  });
+
+  it("renders every theme of both modes, defaults selected", () => {
+    render(<AppearanceSettings />);
+    const pressed = screen
+      .getAllByRole("button", { pressed: true })
+      .map((b) => b.getAttribute("aria-label") ?? b.textContent);
+    expect(pressed).toHaveLength(3); // mode "Dark" + both Default cards
+    for (const name of ["Darcula", "Apple Dark", "Apple Light", "IntelliJ Light", "Canvas"]) {
+      expect(screen.getByText(name)).toBeInTheDocument();
+    }
+  });
+
+  it("picking a dark theme applies it at once; a light pick waits for light mode", async () => {
+    const user = userEvent.setup();
+    render(<AppearanceSettings />);
+    await user.click(screen.getByRole("button", { name: /Darcula/ }));
+    expect(root.dataset.theme).toBe("darcula");
+    await user.click(screen.getByRole("button", { name: /Canvas/ }));
+    expect(root.dataset.theme).toBe("darcula");
+    await user.click(screen.getByRole("button", { name: /^Light$/ }));
+    expect(root.classList.contains("dark")).toBe(false);
+    expect(root.dataset.theme).toBe("canvas");
+    expect(screen.getByRole("button", { name: /Canvas/ })).toHaveAttribute("aria-pressed", "true");
+  });
+
+  it("the System mode button is pressed after choosing it", async () => {
+    const user = userEvent.setup();
+    render(<AppearanceSettings />);
+    await user.click(screen.getByRole("button", { name: /System/ }));
+    expect(screen.getByRole("button", { name: /System/ })).toHaveAttribute("aria-pressed", "true");
+    expect(localStorage.getItem("theme")).toBe("system");
+  });
+
+  it("each swatch scopes its own mode and theme", () => {
+    const { container } = render(<AppearanceSettings />);
+    expect(container.querySelector('.dark[data-theme="darcula"]')).not.toBeNull();
+    expect(container.querySelector('[data-mode="light"][data-theme="canvas"]')).not.toBeNull();
+    expect(container.querySelector('[data-mode="light"]:not([data-theme])')).not.toBeNull();
+  });
+});