refactor(splash): handle Skip redirect in splash screen, decouple SplashButton

Previously, the Skip button (guest login entry) relied on the auth
state machine's `pendingRedirect` flag to trigger the navigation
to /chat after guest login completed. That introduced a
non-obvious coupling: the button dispatched an event, the machine
ran an actor, the auth screen (or splash useEffect) saw the flag
and redirected.

This refactor moves the redirect into the splash screen itself:

- `SplashButton` now takes an `onSkip` prop and no longer knows
  about auth dispatch. Pure presentation component.
- `SplashScreen` provides `handleSkip` that dispatches the
  `AuthGuestLoginSubmitted` event AND immediately calls
  `router.replace(/chat)` for snappier perceived navigation.
- `auth-machine.ts`: `loadingGuestLogin.onDone` no longer sets
  `pendingRedirect: true` (splash already navigated). Sets
  it to `false` explicitly so other screens (auth, sidebar) that
  also react to `pendingRedirect` don't double-navigate if the
  user triggers guest login from a non-splash surface in the future.

No behavior change for the happy path: Skip → /chat works the same.
The refactor is purely about responsibility allocation and
component decoupling.

src/stores/auth/auth-machine.ts

@@ -134,7 +134,8 @@ export const authMachine = setup({
           // 内联 assign —— XState v5 type inference 保留 loadingGuestLogin state 类型
           actions: assign({
             loginStatus: ({ event }) => event.output,
-            pendingRedirect: true, // ← 显式登录成功 → splash / auth 看到跳 /chat
+            // Skip 点击后已在 splash 里立即跳 /chat；guest login 不再依赖 pendingRedirect。
+            pendingRedirect: false,
             errorMessage: null,
           }),
         },