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/user/index.ts

@@ -8,7 +8,8 @@
  *   - Context 形状与初始值位于 user-context.ts
  *   - 事件联合位于 user-events.ts
  */
+export * from "./machine/user-state";
 export * from "./user-context";
-export * from "./user-events";
+export * from "./machine/user-events";
 export * from "./user-types";
 export { userMachine } from "./user-machine";

src/stores/user/machine/user-machine.ts

@@ -17,13 +17,13 @@ import { authRepository } from "@/data/repositories/auth_repository";
 import { UserStorage } from "@/data/storage/user/user_storage";
 import { Result } from "@/utils/result";
 
-import { UserContext, initialContext } from "./user-context";
-import type { UserEvent } from "./user-events";
+import { UserState, initialState } from "./machine/user-state";
+import type { UserEvent } from "./machine/user-events";
 
-// 重新导出 Context / Event 类型，保持 machine 文件的公共 API 不变
-export type { UserContext } from "./user-context";
-export { initialContext } from "./user-context";
-export type { UserEvent } from "./user-events";
+// 重新导出 State / Event 类型，保持 machine 文件的公共 API 不变
+export type { UserState } from "./machine/user-state";
+export { initialState } from "./machine/user-state";
+export type { UserEvent } from "./machine/user-events";
 
 // ============================================================
 // Helpers
@@ -102,7 +102,7 @@ const userLogoutActor = fromPromise<void>(async () => {
 // ============================================================
 export const userMachine = setup({
   types: {
-    context: {} as UserContext,
+    context: {} as UserState,
     events: {} as UserEvent,
   },
   actors: {
@@ -136,7 +136,7 @@ export const userMachine = setup({
 }).createMachine({
   id: "user",
   initial: "idle",
-  context: initialContext,
+  context: initialState,
   states: {
     idle: {
       on: {

src/stores/user/machine/user-state.ts

@@ -1,9 +1,9 @@
 /**
- * User 状态机：Context 形状 + 初始值
+ * User 状态机：State 形状 + 初始值
  */
 import type { UserView } from "@/models/user/user";
 
-export interface UserContext {
+export interface UserState {
   currentUser: UserView | null;
   pronouns: string;
   coinBalance: number;
@@ -11,7 +11,7 @@ export interface UserContext {
   avatarUrl: string | null;
 }
 
-export const initialContext: UserContext = {
+export const initialState: UserState = {
   currentUser: null,
   pronouns: "He",
   coinBalance: 45,

src/stores/user/user-context.tsx

@@ -18,12 +18,12 @@ import {
 import { useMachine } from "@xstate/react";
 
 import { userMachine } from "./user-machine";
-import type { UserContext as MachineContext, UserEvent } from "./user-machine";
+import type { UserState as MachineContext, UserEvent } from "./user-machine";
 
 /**
  * 对外暴露的 State 形状（保持与原 UserState 兼容）
  */
-export interface UserState {
+interface UserState {
   currentUser: MachineContext["currentUser"];
   pronouns: string;
   coinBalance: number;

src/stores/user/user-types.ts

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