refactor(splash): convert Skip to explicit guest login flow

- Splash Skip button now dispatches `AuthGuestLoginSubmitted` instead of
  direct routing, keeping guest auth under the state machine
- Update PWA install dialog copy ("Add to Home Screen") and drop favicon
  entry from manifest icons
- Add debug logging and routing sequence docs to splash-button

src/stores/auth/auth-events.ts

@@ -13,8 +13,10 @@ export type AuthEvent =
   | { type: "AuthModeChanged"; mode: AuthMode }
   | { type: "AuthFormCleared" }
   | { type: "AuthReset" }
-  /** 启动 / 恢复时检查登录态 —— 拿 deviceId → 查 token → 必要时游客登录 */
+  /** 启动 / 恢复时检查登录态 —— 拿 deviceId → 查 token（**不**自动创建游客账号） */
   | { type: "AuthStatusCheckSubmitted" }
+  /** 显式游客登录 —— splash 上点"游客模式"按钮触发（**不**自动） */
+  | { type: "AuthGuestLoginSubmitted" }
   // 业务事件（提交）
   | { type: "AuthEmailLoginSubmitted"; email: string; password: string }
   | {