refactor(sse): 重构 SSE 分层并同步调整通知中心与示例页

- src/composables/sse: 删除，连接实现下沉到 utils/sse，事件名枚举迁到 enums/sse
- src/utils/sse: 新增 SSE 连接与事件订阅实现
- src/stores/dict: 改用 utils/sse 订阅字典变更
- src/stores/user: 登出清理改用 utils/sse 的 cleanupSse
- src/views/dashboard: 在线用户订阅下沉为页面级 composable
- src/components/NoticeCenter: 由 NoticeDropdown 改名
- src/views/demo: 示例页面按 component/route/table 归类
- src/router: 同步示例详情页路由路径

src/router/index.ts

@@ -75,7 +75,7 @@ export const constantRoutes: RouteRecordRaw[] = [
       {
         path: "/detail/:id(\\d+)",
         name: "DemoDetail",
-        component: () => import("@/views/demo/detail.vue"),
+        component: () => import("@/views/demo/route/detail.vue"),
         meta: { title: "详情页缓存", icon: "user", hidden: true, keepAlive: true },
       },
     ],