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/chat/index.ts

@@ -11,7 +11,8 @@
  * 注：chat-side-effects.ts 暂未删除（其中 WebSocket 长生命周期逻辑
  * 待下一轮迁移到 fromCallback actor）。
  */
+export * from "./machine/chat-state";
 export * from "./chat-context";
-export * from "./chat-events";
+export * from "./machine/chat-events";
 export * from "./chat-types";
 export { chatMachine } from "./chat-machine";