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-events.ts

@@ -3,6 +3,7 @@
  *
  * 事件名与原 Dart AuthEvent 对齐。
  */
+import type { AuthProvider } from "@/lib/auth/auth_platform";
 import type { AuthMode } from "@/models/auth/auth-mode";
 import type { AuthPanelMode } from "@/models/auth/auth-panel-mode";
 
@@ -21,7 +22,7 @@ export type AuthEvent =
       username: string;
       confirmPassword: string;
     }
-  // 社交登录（已迁移到 NextAuth）— 业务层直接 await nextauth.googleLogin() 即可
-  | { type: "AuthGoogleLoginSubmitted" }
-  | { type: "AuthFacebookLoginSubmitted" }
+  // 社交登录 —— 状态机内部调 next-auth/react 的 signIn(provider)
+  | { type: "AuthGoogleLoginSubmitted"; provider: AuthProvider }
+  | { type: "AuthFacebookLoginSubmitted"; provider: AuthProvider }
   | { type: "AuthAppleLoginSubmitted" };