refactor(stores): split state and event unions into separate files

Extract `<Name>Context` interface + `initialContext` const into
`<name>-context.ts` and the event union into `<name>-events.ts` for
all four state machines (auth, chat, sidebar, user) under src/stores/.

- `*-machine.ts` keeps helpers, actors, actions, and the
  `setup().createMachine()` body; re-exports the types/initial value
  to preserve its public API.
- `*-types.ts` re-exports from the new files for backward compatibility
  (sidebar keeps enum re-exports, chat keeps GuestChatQuota).
- `index.ts` barrels updated to re-export the new files.
- Removed unused model imports (AuthMode, AuthPanelMode) from
  auth-machine.ts; kept LoginType, UiMessage, UserView where still
  used by actors/helpers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/stores/auth/index.ts

@@ -1,7 +1,12 @@
 /**
- * @file Automatically generated by barrelsby.
+ * @file Auth 公共导出
+ *
+ * 状态机重构后：
+ *   - Context 形状与初始值位于 auth-context.ts
+ *   - 事件联合位于 auth-events.ts
+ *   - machine / 类型 / React 入口分别由对应文件导出
  */
-
 export * from "./auth-context";
+export * from "./auth-events";
 export * from "./auth-machine";
 export * from "./auth-types";