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-context.tsx

@@ -55,9 +55,11 @@ export function AuthProvider({ children }: AuthProviderProps) {
       password: state.context.password,
       username: state.context.username,
       confirmPassword: state.context.confirmPassword,
-      // isLoading 仅来自邮箱登录 actor（社交登录改由 NextAuth 处理，UI 自行管 busy 状态）
-      isLoading: state.matches("loadingEmailLogin") ||
-        state.matches("loadingEmailRegister"),
+      // isLoading 覆盖邮箱登录 / 邮箱注册 / OAuth 跳转（NextAuth 重定向期间）
+      isLoading:
+        state.matches("loadingEmailLogin") ||
+        state.matches("loadingEmailRegister") ||
+        state.matches("loadingOAuth"),
       errorMessage: state.context.errorMessage,
       isSuccess: state.matches("success"),
       loginType: state.context.loginType,