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/music.test.ts

@@ -118,6 +118,25 @@ describe("music router provider gating (enabledProviders) + jellyfin endpoints",
     expect(res.body.enabled).not.toContain("spotify"); // spotify.enabled defaults off
   });
 
+  it("GET /providers reports a configured defaultPlatform override (#126)", async () => {
+    const config = getDefaultConfig();
+    config.defaultPlatform = "qq"; // operator prefers QQ over the priority order
+    const { app } = mount(config);
+    const res = await request(app).get("/api/music/providers");
+    expect(res.status).toBe(200);
+    expect(res.body.default).toBe("qq");
+  });
+
+  it("routes a platform-less /search to the configured defaultPlatform (#126)", async () => {
+    const config = getDefaultConfig();
+    config.defaultPlatform = "bilibili";
+    const { app, netease } = mount(config);
+    const res = await request(app).get("/api/music/search?q=hello");
+    expect(res.status).toBe(200);
+    // Default is now bilibili, so the netease provider must NOT be hit.
+    expect(netease.search).not.toHaveBeenCalled();
+  });
+
   /** Default config plus the opt-in jellyfin source enabled. */
   function configWithJellyfin() {
     const config = getDefaultConfig();