fix(qq): pin @sansenjian/qq-music-api to ~2.4.0 + guard startup failures

A loose `^2.2.10` range let npm pull a newer build of the QQ Music API. The
library went ESM in 2.3.x: an ESM-only 2.3.0/2.3.1 throws ERR_REQUIRE_ESM on
load, so the embedded server never binds port 3200 and every QQ request
(including /getQQLoginQr) fails downstream with ECONNREFUSED — the QR code never
appears and cookies look broken.

- Pin to `~2.4.0` (verified: loads via the bot's native ESM import, exposes the
  Koa app, and every endpoint qq.ts calls returns the exact shapes it parses —
  QR, recommend, lyric, play, playlist detail). Blocks the broken 2.3.0/2.3.1
  and any future 2.5 migration. NOTE: 2.4.x requires Node >=20.17 (or >=22.9).
- Add describeQqApiStartupError(): on startup failure, log an actionable error
  (ERR_REQUIRE_ESM → version-pin hint; engine mismatch → Node-upgrade hint)
  instead of a generic warning, so this is obvious from the logs next time.
- README: troubleshooting entry for "QQ 二维码不弹 / 登录失败 / cookie 无法使用"
  and the version/Node note in the dependency table.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/music/api-server.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect } from "vitest";
+import { describeQqApiStartupError } from "./api-server.js";
+
+describe("describeQqApiStartupError", () => {
+  it("flags ERR_REQUIRE_ESM by error code with version-pin guidance", () => {
+    const hint = describeQqApiStartupError({ code: "ERR_REQUIRE_ESM", message: "..." });
+    expect(hint).toMatch(/ERR_REQUIRE_ESM/);
+    expect(hint).toMatch(/~2\.4\.0/);
+    expect(hint).toMatch(/~2\.2\.10/);
+  });
+
+  it("flags ERR_REQUIRE_ESM by message when the code is absent", () => {
+    const hint = describeQqApiStartupError(
+      new Error("require() of ES Module .../@sansenjian/qq-music-api/dist/index.js not supported")
+    );
+    expect(hint).toMatch(/incompatible @sansenjian\/qq-music-api/);
+  });
+
+  it("flags a Node engine mismatch with a Node-upgrade hint", () => {
+    const hint = describeQqApiStartupError(new Error("Unsupported engine: requires Node >=20.17"));
+    expect(hint).toMatch(/Node >=20\.17/);
+    expect(hint).toMatch(/~2\.2\.10/);
+  });
+
+  it("returns null for an unrelated startup error (falls back to the generic warning)", () => {
+    expect(describeQqApiStartupError(new Error("EADDRINUSE: port in use"))).toBeNull();
+    expect(describeQqApiStartupError(undefined)).toBeNull();
+    expect(describeQqApiStartupError(null)).toBeNull();
+  });
+});