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/chat-events.ts

@@ -1,23 +0,0 @@
-/**
- * Chat 状态机：事件联合
- *
- * 事件名与原 Dart ChatEvent 对齐。
- */
-export type ChatEvent =
-  | { type: "ChatInit" }
-  | { type: "ChatScreenVisible" }
-  | { type: "ChatScreenInvisible" }
-  | { type: "ChatSendMessage"; content: string }
-  | { type: "ChatSendImage"; imageBase64: string }
-  | { type: "ChatLoadMoreHistory" }
-  | {
-      type: "ChatAISentenceReceived";
-      index: number;
-      text: string;
-      total: number;
-      done: boolean;
-    }
-  | { type: "ChatWebSocketError"; errorMessage: string }
-  | { type: "ChatWebSocketConnected"; userId: string }
-  | { type: "ChatQuotaExceeded" }
-  | { type: "ChatAuthStatusChanged" };