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/user/user-events.ts

@@ -0,0 +1,14 @@
+/**
+ * User 状态机：事件联合
+ */
+import type { UserView } from "@/models/user/user";
+
+export type UserEvent =
+  | { type: "UserInit" }
+  | { type: "UserFetch" }
+  | { type: "UserUpdate"; user: UserView }
+  | { type: "UserUpdateUsername"; username: string }
+  | { type: "UserUpdatePronouns"; pronouns: string }
+  | { type: "UserLogout" }
+  | { type: "UserDeleteChatHistory" }
+  | { type: "UserDeleteAccount" };