refactor(auth): unify Facebook and Google login into AuthPlatform class

Replace the separate FacebookLogin and GoogleLogin classes with a single
AuthPlatform class that takes a provider name ("facebook" | "google") as a
constructor argument. This consolidates duplicate OAuth sign-in logic behind
one entry point while preserving the existing NextAuth flow.

- Add new src/lib/auth/auth_platform.ts exporting AuthPlatform and
  AuthProvider type
- Update auth-facebook-panel.tsx and splash-button.tsx to use the new API
- Rename initialContext → initialState in the auth machine for consistency
- Update inline docs and comments to reference AuthPlatform

No behavioral change for end users; both providers still route through
NextAuth's signIn() with the same callbacks and cookies.

src/stores/auth/auth-state.ts

@@ -1,11 +1,11 @@
 /**
- * Auth 状态机：Context 形状 + 初始值
+ * Auth 状态机：State 形状 + 初始值
  */
 import type { AuthMode } from "@/models/auth/auth-mode";
 import type { AuthPanelMode } from "@/models/auth/auth-panel-mode";
 import type { LoginType } from "@/models/auth/login-type";
 
-export interface AuthContext {
+export interface AuthState {
   /** 当前面板模式（Facebook / Email） */
   authPanelMode: AuthPanelMode;
   /** 内部登录模式（login / register） */
@@ -18,7 +18,7 @@ export interface AuthContext {
   loginType: LoginType;
 }
 
-export const initialContext: AuthContext = {
+export const initialState: AuthState = {
   authPanelMode: "facebook",
   authMode: "login",
   email: "",

src/stores/auth/auth-types.ts

@@ -7,4 +7,4 @@
  *   - Context 形状与初始值由 `auth-context.ts` 导出
  *   - `AuthState` 由 `auth-context.tsx` 导出（保持原 API 兼容）
  */
-export type { AuthEvent } from "./auth-events";
+export type { AuthEvent } from "./machine/auth-events";

src/stores/auth/index.ts

@@ -6,7 +6,8 @@
  *   - 事件联合位于 auth-events.ts
  *   - machine / 类型 / React 入口分别由对应文件导出
  */
+export * from "./auth-state";
 export * from "./auth-context";
-export * from "./auth-events";
-export * from "./auth-machine";
+export * from "./machine/auth-events";
+export * from "./machine/auth-machine";
 export * from "./auth-types";

src/stores/auth/machine/auth-context.tsx

@@ -18,12 +18,12 @@ import {
 import { useMachine } from "@xstate/react";
 
 import { authMachine } from "./auth-machine";
-import type { AuthEvent, AuthContext as MachineContext } from "./auth-machine";
+import type { AuthEvent, AuthState as MachineContext } from "./auth-machine";
 
 /**
  * 对外暴露的 State 形状（保持与原 AuthState 兼容）
  */
-export interface AuthState {
+interface AuthState {
   authPanelMode: MachineContext["authPanelMode"];
   authMode: MachineContext["authMode"];
   email: string;

src/stores/auth/machine/auth-machine.ts

@@ -4,8 +4,8 @@
  * 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 GoogleLogin().signIn()` /
- * `new FacebookLogin().signIn()`。
+ * 邮箱注册/登录 + 通用 reset 事件。社交登录走 `new AuthPlatform("google").signIn()` /
+ * `new AuthPlatform("facebook").signIn()`。
  */
 import { setup, fromPromise, assign } from "xstate";
 
@@ -14,12 +14,12 @@ import { authRepository } from "@/data/repositories/auth_repository";
 import { AuthStorage } from "@/data/storage/auth/auth_storage";
 import { Result } from "@/utils/result";
 
-import { AuthContext, initialContext } from "./auth-context";
+import { AuthState, initialState } from "../auth-state";
 import type { AuthEvent } from "./auth-events";
 
-// 重新导出 Context / Event 类型，保持 machine 文件的公共 API 不变
-export type { AuthContext } from "./auth-context";
-export { initialContext } from "./auth-context";
+// 重新导出 State / Event 类型，保持 machine 文件的公共 API 不变
+export type { AuthState } from "../auth-state";
+export { initialState } from "../auth-state";
 export type { AuthEvent } from "./auth-events";
 
 // ============================================================
@@ -73,7 +73,7 @@ const emailRegisterThenLoginActor = fromPromise<
 // ============================================================
 export const authMachine = setup({
   types: {
-    context: {} as AuthContext,
+    context: {} as AuthState,
     events: {} as AuthEvent,
   },
   actors: {
@@ -83,7 +83,7 @@ export const authMachine = setup({
 }).createMachine({
   id: "auth",
   initial: "idle",
-  context: initialContext,
+  context: initialState,
   states: {
     idle: {
       on: {
@@ -109,11 +109,11 @@ export const authMachine = setup({
           }),
         },
         AuthReset: {
-          actions: assign(() => initialContext),
+          actions: assign(() => initialState),
         },
         AuthEmailLoginSubmitted: "loadingEmailLogin",
         AuthEmailRegisterSubmitted: "loadingEmailRegister",
-        // 社交登录已迁移到 NextAuth + GoogleLogin / FacebookLogin 类
+        // 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: {},