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/hooks/__tests__/useTheme.test.ts

@@ -1,10 +1,10 @@
 /**
  * @file useTheme.test.ts
- * @description Tests for useTheme — the dark/light mode hook.
+ * @description Tests for useTheme — the dark/light/system mode and per-mode named-theme hook.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
 import { renderHook, act } from "@testing-library/react";
 import { useTheme } from "../useTheme";
 
@@ -12,9 +12,12 @@ describe("useTheme", () => {
   beforeEach(() => {
     localStorage.clear();
     document.documentElement.classList.remove("dark");
+    document.documentElement.removeAttribute("data-theme");
+    document.documentElement.removeAttribute("data-mode");
   });
   afterEach(() => {
     document.documentElement.classList.remove("dark");
+    vi.unstubAllGlobals();
   });
 
   it("defaults to dark when localStorage is empty", () => {
@@ -46,4 +49,165 @@ describe("useTheme", () => {
     expect(result.current.theme).toBe("dark");
     expect(document.documentElement.classList.contains("dark")).toBe(true);
   });
+
+  it("system mode follows prefers-color-scheme, live", () => {
+    let dark = true;
+    const listeners = new Set<() => void>();
+    vi.stubGlobal("matchMedia", (query: string) => ({
+      get matches() {
+        return dark;
+      },
+      media: query,
+      addEventListener: (_: string, fn: () => void) => listeners.add(fn),
+      removeEventListener: (_: string, fn: () => void) => listeners.delete(fn),
+    }));
+    const { result } = renderHook(() => useTheme());
+    act(() => result.current.setMode("system"));
+    expect(result.current.mode).toBe("system");
+    expect(result.current.theme).toBe("dark");
+    expect(localStorage.getItem("theme")).toBe("system");
+    act(() => {
+      dark = false;
+      listeners.forEach((fn) => fn());
+    });
+    expect(result.current.theme).toBe("light");
+    expect(document.documentElement.classList.contains("dark")).toBe(false);
+    expect(document.documentElement.dataset.mode).toBe("light");
+  });
+
+  it("system mode without matchMedia falls back to dark", () => {
+    vi.stubGlobal("matchMedia", undefined);
+    localStorage.setItem("theme", "system");
+    const { result } = renderHook(() => useTheme());
+    expect(result.current.theme).toBe("dark");
+  });
+
+  it("each mode remembers its own named theme", () => {
+    const { result } = renderHook(() => useTheme());
+    act(() => result.current.setThemeFor("dark", "darcula"));
+    act(() => result.current.setThemeFor("light", "canvas"));
+    expect(document.documentElement.dataset.theme).toBe("darcula");
+    expect(localStorage.getItem("agenthub.theme.dark")).toBe("darcula");
+    act(() => result.current.toggleTheme());
+    expect(result.current.theme).toBe("light");
+    expect(document.documentElement.dataset.theme).toBe("canvas");
+    expect(result.current.themeFor).toEqual({ dark: "darcula", light: "canvas" });
+  });
+
+  it("the default theme carries no data-theme attribute", () => {
+    const { result } = renderHook(() => useTheme());
+    act(() => result.current.setThemeFor("dark", "darcula"));
+    act(() => result.current.setThemeFor("dark", "default"));
+    expect(document.documentElement.hasAttribute("data-theme")).toBe(false);
+  });
+
+  it("an unknown stored theme id falls back to default", () => {
+    localStorage.setItem("agenthub.theme.dark", "no-such-theme");
+    const { result } = renderHook(() => useTheme());
+    expect(result.current.themeFor.dark).toBe("default");
+    expect(document.documentElement.hasAttribute("data-theme")).toBe(false);
+  });
+
+  it("a light theme id is never applied in dark mode", () => {
+    const { result } = renderHook(() => useTheme());
+    act(() => result.current.setThemeFor("dark", "canvas"));
+    expect(result.current.themeFor.dark).toBe("default");
+    expect(document.documentElement.hasAttribute("data-theme")).toBe(false);
+  });
+
+  it("two hook instances stay in sync", () => {
+    const a = renderHook(() => useTheme());
+    const b = renderHook(() => useTheme());
+    act(() => a.result.current.setTheme("light"));
+    act(() => a.result.current.setThemeFor("light", "apple-light"));
+    expect(b.result.current.theme).toBe("light");
+    expect(b.result.current.themeFor.light).toBe("apple-light");
+  });
+
+  it("another tab's storage write updates a mounted hook", () => {
+    const { result } = renderHook(() => useTheme());
+    expect(result.current.theme).toBe("dark");
+    act(() => {
+      localStorage.setItem("theme", "light");
+      localStorage.setItem("agenthub.theme.light", "canvas");
+      window.dispatchEvent(new StorageEvent("storage", { key: "theme" }));
+    });
+    expect(result.current.theme).toBe("light");
+    expect(document.documentElement.dataset.theme).toBe("canvas");
+    expect(document.documentElement.dataset.mode).toBe("light");
+  });
+
+  it("disabled storage (throwing) falls back to dark/default and setters do not crash", () => {
+    const get = vi.spyOn(Storage.prototype, "getItem").mockImplementation(() => {
+      throw new Error("SecurityError");
+    });
+    const set = vi.spyOn(Storage.prototype, "setItem").mockImplementation(() => {
+      throw new Error("QuotaExceededError");
+    });
+    try {
+      const { result } = renderHook(() => useTheme());
+      expect(result.current.theme).toBe("dark");
+      expect(result.current.mode).toBe("dark");
+      expect(result.current.themeFor).toEqual({ dark: "default", light: "default" });
+      expect(() => act(() => result.current.setMode("light"))).not.toThrow();
+      expect(() => act(() => result.current.setThemeFor("dark", "darcula"))).not.toThrow();
+      expect(() => act(() => result.current.toggleTheme())).not.toThrow();
+      expect(result.current.theme).toBe("dark");
+    } finally {
+      get.mockRestore();
+      set.mockRestore();
+    }
+  });
+
+  it("setThemeFor with an unknown id keeps the previous pick", () => {
+    const { result } = renderHook(() => useTheme());
+    act(() => result.current.setThemeFor("dark", "darcula"));
+    act(() => result.current.setThemeFor("dark", "no-such-theme"));
+    act(() => result.current.setThemeFor("light", ""));
+    expect(result.current.themeFor).toEqual({ dark: "darcula", light: "default" });
+    expect(localStorage.getItem("agenthub.theme.dark")).toBe("darcula");
+    expect(localStorage.getItem("agenthub.theme.light")).toBeNull();
+    expect(document.documentElement.dataset.theme).toBe("darcula");
+  });
+
+  it("an unknown stored mode falls back to dark", () => {
+    localStorage.setItem("theme", "blue");
+    const { result } = renderHook(() => useTheme());
+    expect(result.current.mode).toBe("dark");
+    expect(result.current.theme).toBe("dark");
+    expect(document.documentElement.dataset.mode).toBe("dark");
+  });
+
+  it.each([
+    [true, "light"],
+    [false, "dark"],
+  ])("toggle from system (OS dark=%s) sets the explicit opposite: %s", (osDark, expected) => {
+    vi.stubGlobal("matchMedia", (query: string) => ({
+      matches: osDark,
+      media: query,
+      addEventListener: () => {},
+      removeEventListener: () => {},
+    }));
+    localStorage.setItem("theme", "system");
+    const { result } = renderHook(() => useTheme());
+    expect(result.current.mode).toBe("system");
+    act(() => result.current.toggleTheme());
+    expect(result.current.mode).toBe(expected);
+    expect(result.current.theme).toBe(expected);
+    expect(localStorage.getItem("theme")).toBe(expected);
+  });
+
+  it("unmount removes the matchMedia listener", () => {
+    const listeners = new Set<() => void>();
+    vi.stubGlobal("matchMedia", (query: string) => ({
+      matches: true,
+      media: query,
+      addEventListener: (_: string, fn: () => void) => listeners.add(fn),
+      removeEventListener: (_: string, fn: () => void) => listeners.delete(fn),
+    }));
+    const { unmount } = renderHook(() => useTheme());
+    expect(listeners.size).toBeGreaterThan(0);
+    unmount();
+    expect(listeners.size).toBe(0);
+  });
 });