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-context.tsx

@@ -20,13 +20,13 @@ import {
 } from "react";
 import { useMachine } from "@xstate/react";
 
-import { chatMachine } from "./chat-machine";
-import type { ChatEvent, ChatContext as MachineContext } from "./chat-machine";
+import { chatMachine } from "./machine/chat-machine";
+import type { ChatEvent, ChatState as MachineContext } from "./machine/chat-machine";
 
 /**
  * 对外暴露的 State 形状（保持与原 ChatState 兼容）
  */
-export interface ChatState {
+interface ChatState {
   messages: MachineContext["messages"];
   isReplyingAI: boolean;
   isGuest: boolean;

src/stores/chat/chat-types.ts

@@ -8,5 +8,5 @@
  *   - `ChatState` 由 `chat-context.tsx` 导出（保持原 API 兼容）
  *   - `GuestChatQuota` 仍由 `chat-machine.ts` 导出（业务常量，与上下文配对紧密）
  */
-export type { ChatEvent } from "./chat-events";
+export type { ChatEvent } from "./machine/chat-events";
 export { GuestChatQuota } from "./chat-machine";

src/stores/chat/index.ts

@@ -11,7 +11,8 @@
  * 注：chat-side-effects.ts 暂未删除（其中 WebSocket 长生命周期逻辑
  * 待下一轮迁移到 fromCallback actor）。
  */
+export * from "./machine/chat-state";
 export * from "./chat-context";
-export * from "./chat-events";
+export * from "./machine/chat-events";
 export * from "./chat-types";
 export { chatMachine } from "./chat-machine";

src/stores/chat/machine/chat-machine.ts

@@ -23,8 +23,8 @@ import { AuthStorage } from "@/data/storage/auth/auth_storage";
 import { formatDate } from "@/utils/date";
 import { Result } from "@/utils/result";
 
-import { ChatContext, initialContext } from "./chat-context";
-import type { ChatEvent } from "./chat-events";
+import { ChatState, initialState } from "./chat-state";
+import type { ChatEvent } from "./machine/chat-events";
 
 /** 游客配额阈值（与 Dart `GuestChatQuota` 保持一致） */
 export const GuestChatQuota = {
@@ -32,10 +32,10 @@ export const GuestChatQuota = {
   quotaExhausted: 0,
 } as const;
 
-// 重新导出 Context / Event 类型，保持 machine 文件的公共 API 不变
-export type { ChatContext } from "./chat-context";
-export { initialContext } from "./chat-context";
-export type { ChatEvent } from "./chat-events";
+// 重新导出 State / Event 类型，保持 machine 文件的公共 API 不变
+export type { ChatState } from "./chat-state";
+export { initialState } from "./chat-state";
+export type { ChatEvent } from "./machine/chat-events";
 
 // ============================================================
 // Helpers
@@ -146,7 +146,7 @@ const loadMoreHistoryActor = fromPromise<
 // ============================================================
 export const chatMachine = setup({
   types: {
-    context: {} as ChatContext,
+    context: {} as ChatState,
     events: {} as ChatEvent,
   },
   actors: {
@@ -230,7 +230,7 @@ export const chatMachine = setup({
 }).createMachine({
   id: "chat",
   initial: "idle",
-  context: initialContext,
+  context: initialState,
   states: {
     idle: {
       on: {

src/stores/chat/machine/chat-state.ts

@@ -1,11 +1,11 @@
 /**
- * Chat 状态机：Context 形状 + 初始值
+ * Chat 状态机：State 形状 + 初始值
  *
  * 注：GuestChatQuota 仍位于 chat-machine.ts（属于业务常量，与上下文配对紧密）。
  */
 import type { UiMessage } from "@/models/chat/ui-message";
 
-export interface ChatContext {
+export interface ChatState {
   messages: UiMessage[];
   isReplyingAI: boolean;
   isGuest: boolean;
@@ -17,7 +17,7 @@ export interface ChatContext {
   historyOffset: number;
 }
 
-export const initialContext: ChatContext = {
+export const initialState: ChatState = {
   messages: [],
   isReplyingAI: false,
   isGuest: true,