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-context.tsx

@@ -1,86 +0,0 @@
-"use client";
-/**
- * AuthContext：基于 XState v5 的 React Context Provider
- *
- * 业务事件（`Auth*Submitted`）通过 `useMachine(authMachine).send()` 派发。
- * XState 自动处理异步副作用（invoke + fromPromise actors）。
- *
- * 兼容性：保持原 `useAuthState()` / `useAuthDispatch()` API，
- * 内部将 XState 状态机快照映射回原 `AuthState` 形状。
- */
-import {
-  type Dispatch,
-  type ReactNode,
-  createContext,
-  useContext,
-  useMemo,
-} from "react";
-import { useMachine } from "@xstate/react";
-
-import { authMachine } from "./auth-machine";
-import type { AuthEvent, AuthContext as MachineContext } from "./auth-machine";
-
-/**
- * 对外暴露的 State 形状（保持与原 AuthState 兼容）
- */
-export interface AuthState {
-  authPanelMode: MachineContext["authPanelMode"];
-  authMode: MachineContext["authMode"];
-  email: string;
-  password: string;
-  username: string;
-  confirmPassword: string;
-  isLoading: boolean;
-  errorMessage: string | null;
-  isSuccess: boolean;
-  loginType: MachineContext["loginType"];
-}
-
-const AuthStateCtx = createContext<AuthState | null>(null);
-const AuthDispatchCtx = createContext<Dispatch<AuthEvent> | null>(null);
-
-export interface AuthProviderProps {
-  children: ReactNode;
-}
-
-export function AuthProvider({ children }: AuthProviderProps) {
-  const [state, send] = useMachine(authMachine);
-
-  // 映射 XState 状态机快照 → 原 AuthState 形状
-  const authState = useMemo<AuthState>(
-    () => ({
-      authPanelMode: state.context.authPanelMode,
-      authMode: state.context.authMode,
-      email: state.context.email,
-      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"),
-      errorMessage: state.context.errorMessage,
-      isSuccess: state.matches("success"),
-      loginType: state.context.loginType,
-    }),
-    [state],
-  );
-
-  return (
-    <AuthStateCtx.Provider value={authState}>
-      <AuthDispatchCtx.Provider value={send}>{children}</AuthDispatchCtx.Provider>
-    </AuthStateCtx.Provider>
-  );
-}
-
-export function useAuthState(): AuthState {
-  const ctx = useContext(AuthStateCtx);
-  if (!ctx) throw new Error("useAuthState must be used inside <AuthProvider>");
-  return ctx;
-}
-
-export function useAuthDispatch(): Dispatch<AuthEvent> {
-  const ctx = useContext(AuthDispatchCtx);
-  if (!ctx)
-    throw new Error("useAuthDispatch must be used inside <AuthProvider>");
-  return ctx;
-}