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/index.ts

@@ -8,7 +8,8 @@
  *   - Context 形状与初始值位于 sidebar-context.ts
  *   - 事件联合位于 sidebar-events.ts
  */
+export * from "./sidebar-state";
 export * from "./sidebar-context";
 export * from "./sidebar-events";
 export * from "./sidebar-types";
-export { sidebarMachine } from "./sidebar-machine";
+export { sidebarMachine } from "./machine/sidebar-machine";