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/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;