refactor(auth): funnel OAuth signIn into auth state machine

Move new AuthPlatform(provider).signIn() out of UI components and into
the auth state machine. UI now dispatches AuthGoogle/FacebookLoginSubmitted
events; a new oauthSignIn actor inside the machine calls next-auth/react's
signIn(provider). Errors and the OAuth redirect window are exposed via
the existing state.isLoading / state.errorMessage surface, removing the
local busy / error state in auth-facebook-panel.

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

src/stores/auth/auth-machine.ts

@@ -3,12 +3,14 @@
  *
  * 原始 Dart: lib/ui/auth/bloc/auth_bloc.dart + auth_state.dart + auth_event.dart
  *
- * 本轮迁移：社交登录改由 NextAuth 处理（`@/lib/auth/nextauth`），状态机仅保留
- * 邮箱注册/登录 + 通用 reset 事件。社交登录走 `new AuthPlatform("google").signIn()` /
- * `new AuthPlatform("facebook").signIn()`。
+ * 本轮迁移：所有认证登录调用（含 OAuth 社交登录）统一收口到状态机内。
+ * 业务层只派发事件（`AuthEmailLoginSubmitted` / `AuthGoogleLoginSubmitted` / 等），
+ * 状态机通过 actors 调 `authRepository`（邮箱）或 `next-auth/react.signIn`（OAuth）。
  */
 import { setup, fromPromise, assign } from "xstate";
+import { signIn } from "next-auth/react";
 
+import type { AuthProvider } from "@/lib/auth/auth_platform";
 import type { LoginType } from "@/models/auth/login-type";
 import { authRepository } from "@/data/repositories/auth_repository";
 import { AuthStorage } from "@/data/storage/auth/auth_storage";
@@ -68,6 +70,13 @@ const emailRegisterThenLoginActor = fromPromise<
   return "email" as LoginType;
 });
 
+// OAuth 登录 actor：调 next-auth/react 的 signIn(provider) 触发 OAuth 跳转。
+// 成功路径下 NextAuth 会重定向离开本应用，actor 通常不会 resolve；
+// onDone 仍保留以处理 SDK 同步返回（极少见）。
+const oauthSignInActor = fromPromise<void, AuthProvider>(async ({ input }) => {
+  await signIn(input);
+});
+
 // ============================================================
 // Machine
 // ============================================================
@@ -79,6 +88,7 @@ export const authMachine = setup({
   actors: {
     emailLogin: emailLoginActor,
     emailRegisterThenLogin: emailRegisterThenLoginActor,
+    oauthSignIn: oauthSignInActor,
   },
 }).createMachine({
   id: "auth",
@@ -113,10 +123,8 @@ export const authMachine = setup({
         },
         AuthEmailLoginSubmitted: "loadingEmailLogin",
         AuthEmailRegisterSubmitted: "loadingEmailRegister",
-        // 社交登录已迁移到 NextAuth + AuthPlatform 类
-        // 状态机保留事件以兼容旧调用方（实际不再触发）
-        AuthGoogleLoginSubmitted: {},
-        AuthFacebookLoginSubmitted: {},
+        AuthGoogleLoginSubmitted: "loadingOAuth",
+        AuthFacebookLoginSubmitted: "loadingOAuth",
         AuthAppleLoginSubmitted: {
           actions: assign({
             errorMessage: "Apple login not implemented",
@@ -180,6 +188,31 @@ export const authMachine = setup({
       },
     },
 
+    loadingOAuth: {
+      entry: assign({ errorMessage: null }),
+      invoke: {
+        src: "oauthSignIn",
+        input: ({ event }) => {
+          // event 来自 idle 跳转（AuthGoogle/FacebookLoginSubmitted）
+          if (event.type === "AuthGoogleLoginSubmitted") return event.provider;
+          if (event.type === "AuthFacebookLoginSubmitted") return event.provider;
+          return "google" as AuthProvider;
+        },
+        onDone: {
+          // 罕见分支：NextAuth 同步 resolve（实际几乎不会发生，
+          // OAuth 流程会通过重定向离开本应用）。
+          target: "idle",
+        },
+        onError: {
+          target: "idle",
+          actions: assign({
+            errorMessage: ({ event }) =>
+              event.error instanceof Error ? event.error.message : String(event.error),
+          }),
+        },
+      },
+    },
+
     success: {
       // 终态：业务层通过 state.matches("success") 判断
     },