feat: edit in the Changes panel, git graph history, plan quota (v2.3.0)

- Changes panel: a working file opens in a CodeMirror editor with its diff
  inline (DiffEditor.tsx, @codemirror/merge, lazy-loaded); Changes folds the
  unchanged lines, Whole file shows them; side by side at 50/50 and full
  width; a Revert per block; original from HEAD or the branch's merge base
  (`mergeBase` on GET /git/changes?range=branch)
- Every diff: green = line added whole, red = removed whole, blue = edited
  in place (--diff-mod)
- History drawn as a commit graph (#58: `parents` on GET /git/log,
  --date-order; lib/commitGraph.ts)
- Per-workspace never-stage list, drawn orange in the Files tree (--kept);
  closed Terminal rows past three deleted
- Status bar plan quota for Claude (per account) and Codex
  (GET /api/pricing/quota, server/lib/quota.js); compact hover panel that
  stays open, Refresh button with a spinner
- Settings → Appearance holds language, theme and pet; Ctrl+Shift+C copies;
  compact sidebar navigation and short session times

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

@@ -1,14 +1,17 @@
 /**
  * @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>`.
+ * @description Tests for the Settings → Appearance section: the language switch, and the mode switch and the per-mode theme cards writing 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 { describe, it, expect, beforeEach, vi } from "vitest";
+import { act, render, screen, within } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
+import i18n from "../../../i18n";
 import { AppearanceSettings } from "../AppearanceSettings";
 
+vi.mock("../PetSettings", () => ({ PetSettings: () => <div data-testid="pet-settings" /> }));
+
 const root = document.documentElement;
 
 describe("AppearanceSettings", () => {
@@ -19,6 +22,22 @@ describe("AppearanceSettings", () => {
     root.removeAttribute("data-mode");
   });
 
+  it("switches the interface language and holds the pet block", async () => {
+    const user = userEvent.setup();
+    render(<AppearanceSettings />);
+    expect(screen.getByTestId("pet-settings").closest("section")).toHaveAttribute("id", "pet");
+    const group = screen.getByRole("radiogroup", { name: "Interface language" });
+    expect(within(group).getByRole("radio", { name: "English" })).toHaveAttribute(
+      "aria-checked",
+      "true"
+    );
+    await user.click(within(group).getByRole("radio", { name: "Tiếng Việt" }));
+    expect(i18n.language).toBe("vi");
+    await act(async () => {
+      await i18n.changeLanguage("en");
+    });
+  });
+
   it("renders every theme of both modes, defaults selected", () => {
     render(<AppearanceSettings />);
     const pressed = screen