feat: add configurable default music source (#126)

Make the default playback source a user-configurable setting so servers
that mostly play e.g. Bilibili no longer need to type `-b` on every
`!play`. Previously defaultPlatform() always picked the first enabled
provider by a fixed priority order, with no way to override it.

- config: add optional `defaultPlatform: GateableProvider | null`.
  loadConfig sanitizes it — kept only when it names a known provider that
  is also currently enabled, else null. defaultPlatform() returns the
  preference when enabled, otherwise falls back to the fixed priority order.
- POST /api/bot/settings accepts `defaultPlatform` (validated against the
  possibly-updated enabledProviders; null/"" clears it), and reconciles a
  stored default that a new enabledProviders list no longer allows. GET and
  POST responses expose the field.
- WebUI: new "默认音源" section with a source picker; saving refreshes the
  store's default source so it takes effect immediately without a restart.
- Tests: extend config defaultPlatform priority tests and add coverage for
  the settings endpoint and /providers routing.
- README: document `defaultPlatform` in the enabledProviders section.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/api/bot.test.ts

@@ -647,4 +647,54 @@ describe("bot router /settings jellyfin block + enabledProviders", () => {
     // No jellyfin block in the request → no reconfigure call.
     expect(configureCalls).toHaveLength(0);
   });
+
+  // --- #126: operator-chosen default source ---
+
+  it("GET /settings exposes defaultPlatform (null by default)", async () => {
+    const res = await request(mountBot()).get("/api/bot/settings");
+    expect(res.status).toBe(200);
+    expect(res.body.defaultPlatform).toBeNull();
+  });
+
+  it("POST /settings sets an enabled defaultPlatform and persists it", async () => {
+    const res = await request(mountBot()).post("/api/bot/settings").send({
+      defaultPlatform: "bilibili",
+    });
+    expect(res.status).toBe(200);
+    expect(res.body.defaultPlatform).toBe("bilibili");
+    expect(config.defaultPlatform).toBe("bilibili");
+    const onDisk = JSON.parse(readFileSync(configPath, "utf-8"));
+    expect(onDisk.defaultPlatform).toBe("bilibili");
+  });
+
+  it("POST /settings ignores an unknown or disabled defaultPlatform", async () => {
+    const app = mountBot();
+    // jellyfin is opt-in and not enabled in the default config → rejected.
+    await request(app).post("/api/bot/settings").send({ defaultPlatform: "jellyfin" });
+    expect(config.defaultPlatform).toBeNull();
+    // Unknown value → rejected.
+    await request(app).post("/api/bot/settings").send({ defaultPlatform: "bogus" });
+    expect(config.defaultPlatform).toBeNull();
+  });
+
+  it("POST /settings clears defaultPlatform with null", async () => {
+    const app = mountBot();
+    await request(app).post("/api/bot/settings").send({ defaultPlatform: "qq" });
+    expect(config.defaultPlatform).toBe("qq");
+    const res = await request(app).post("/api/bot/settings").send({ defaultPlatform: null });
+    expect(res.body.defaultPlatform).toBeNull();
+    expect(config.defaultPlatform).toBeNull();
+  });
+
+  it("POST /settings drops a default whose source gets disabled in the same request", async () => {
+    const app = mountBot();
+    await request(app).post("/api/bot/settings").send({ defaultPlatform: "qq" });
+    expect(config.defaultPlatform).toBe("qq");
+    // Disabling qq via enabledProviders clears the now-invalid default.
+    const res = await request(app).post("/api/bot/settings").send({
+      enabledProviders: ["netease", "bilibili"],
+    });
+    expect(res.body.defaultPlatform).toBeNull();
+    expect(config.defaultPlatform).toBeNull();
+  });
 });