refactor(auth): remove dead AuthReset event

AuthReset was declared in the AuthEvent union and handled in
auth-machine.idle.on with `assign(() => initialState)`, but
nothing in the codebase actually dispatches it anymore.

History of dispatch sites (now all gone):
- sidebar-screen.tsx previously dispatched AuthReset post-logout
  alongside router.replace(ROUTES.chat) — that was simplified to
  just dispatch AuthLogoutSubmitted, the state machine's
  loggingOut state now does the context reset itself in onDone
  via the same `assign(() => initialState)` action.
- chat-screen / splash / auth screens never dispatched it.
- The /auth screen's pendingRedirect-based redirect loop doesn't
  need it (the redirect is one-shot, not a state that needs
  resetting).

So the event handler is unreachable. Remove the type member
from auth-events.ts and the handler from auth-machine.ts.

No behavior change: the only path that 'reset auth state' was
already covered by loggingOut.onDone.

Verified via `grep -rn 'AuthReset' src/`: only 2 hits remain
after this commit — both are the type declaration and the
machine handler, which is now also gone (zero hits).

src/stores/auth/auth-events.ts

@@ -12,7 +12,6 @@ export type AuthEvent =
   | { type: "AuthPanelModeChanged"; mode: AuthPanelMode }
   | { type: "AuthModeChanged"; mode: AuthMode }
   | { type: "AuthFormCleared" }
-  | { type: "AuthReset" }
   | { type: "AuthLogoutSubmitted" }
   /** App 启动时一次性派发 —— 读 storage 把 loginStatus 同步到状态机 */
   | { type: "AuthInit" }

src/stores/auth/auth-machine.ts

@@ -78,9 +78,6 @@ export const authMachine = setup({
             errorMessage: null,
           }),
         },
-        AuthReset: {
-          actions: assign(() => initialState),
-        },
         AuthLogoutSubmitted: "loggingOut",
         // 启动一次性 init：从 storage 同步 loginStatus 到状态机 —— 由 <AuthStatusChecker /> 派发
         AuthInit: "initializing",