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/sidebar/sidebar-context.tsx

@@ -17,16 +17,16 @@ import {
 } from "react";
 import { useMachine } from "@xstate/react";
 
-import { sidebarMachine } from "./sidebar-machine";
+import { sidebarMachine } from "./machine/sidebar-machine";
 import type {
-  SidebarContext as MachineContext,
+  SidebarState as MachineContext,
   SidebarEvent,
-} from "./sidebar-machine";
+} from "./machine/sidebar-machine";
 
 /**
  * 对外暴露的 State 形状（保持与原 SidebarState 兼容）
  */
-export interface SidebarState {
+interface SidebarState {
   currentPage: MachineContext["currentPage"];
   selectedBottomNavItem: MachineContext["selectedBottomNavItem"];
   characterName: string;