Fix: 修复终端标签切换时重连问题

将终端工作区提升为主布局常驻层，离开终端路由时只隐藏不卸载组件。
新增活动终端显隐状态跟踪，页面恢复时自动重新适配尺寸和聚焦。

改动范围：
- frontend/src/layouts/MainLayout.vue
- frontend/src/views/TerminalWorkspaceView.vue
- frontend/src/components/TerminalWidget.vue

frontend/src/views/TerminalWorkspaceView.vue

@@ -4,6 +4,10 @@ import { useTerminalTabsStore } from '../stores/terminalTabs'
 import { useConnectionsStore } from '../stores/connections'
 import TerminalWidget from '../components/TerminalWidget.vue'
 
+const props = defineProps<{
+  visible?: boolean
+}>()
+
 const tabsStore = useTerminalTabsStore()
 const connectionsStore = useConnectionsStore()
 
@@ -34,7 +38,10 @@ onMounted(() => {
           v-show="tab.active"
           class="h-full"
         >
-          <TerminalWidget :connection-id="tab.connectionId" />
+          <TerminalWidget
+            :connection-id="tab.connectionId"
+            :active="tab.active && props.visible !== false"
+          />
         </div>
       </div>
     </div>