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-events.ts

@@ -1,14 +0,0 @@
-/**
- * User 状态机：事件联合
- */
-import type { UserView } from "@/models/user/user";
-
-export type UserEvent =
-  | { type: "UserInit" }
-  | { type: "UserFetch" }
-  | { type: "UserUpdate"; user: UserView }
-  | { type: "UserUpdateUsername"; username: string }
-  | { type: "UserUpdatePronouns"; pronouns: string }
-  | { type: "UserLogout" }
-  | { type: "UserDeleteChatHistory" }
-  | { type: "UserDeleteAccount" };