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-state.ts

@@ -0,0 +1,30 @@
+/**
+ * Auth 状态机：State 形状 + 初始值
+ */
+import type { AuthMode } from "@/models/auth/auth-mode";
+import type { AuthPanelMode } from "@/models/auth/auth-panel-mode";
+import type { LoginType } from "@/models/auth/login-type";
+
+export interface AuthState {
+  /** 当前面板模式（Facebook / Email） */
+  authPanelMode: AuthPanelMode;
+  /** 内部登录模式（login / register） */
+  authMode: AuthMode;
+  email: string;
+  password: string;
+  username: string;
+  confirmPassword: string;
+  errorMessage: string | null;
+  loginType: LoginType;
+}
+
+export const initialState: AuthState = {
+  authPanelMode: "facebook",
+  authMode: "login",
+  email: "",
+  password: "",
+  username: "",
+  confirmPassword: "",
+  errorMessage: null,
+  loginType: "none",
+};