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/layouts/MainLayout.vue

@@ -1,19 +1,22 @@
-<script setup lang="ts">
-import { ref, computed } from 'vue'
-import { RouterLink, useRoute, useRouter } from 'vue-router'
-import { useAuthStore } from '../stores/auth'
-import { useConnectionsStore } from '../stores/connections'
-import { useTerminalTabsStore } from '../stores/terminalTabs'
-import { ArrowLeftRight, Server, LogOut, Menu, X, Terminal } from 'lucide-vue-next'
+<script setup lang="ts">
+import { ref, computed } from 'vue'
+import { RouterLink, useRoute, useRouter } from 'vue-router'
+import { useAuthStore } from '../stores/auth'
+import { useConnectionsStore } from '../stores/connections'
+import { useTerminalTabsStore } from '../stores/terminalTabs'
+import TerminalWorkspaceView from '../views/TerminalWorkspaceView.vue'
+import { ArrowLeftRight, Server, LogOut, Menu, X, Terminal } from 'lucide-vue-next'
 
 const route = useRoute()
 const router = useRouter()
 const authStore = useAuthStore()
 const connectionsStore = useConnectionsStore()
 const tabsStore = useTerminalTabsStore()
-const sidebarOpen = ref(false)
-
-const terminalTabs = computed(() => tabsStore.tabs)
+const sidebarOpen = ref(false)
+
+const terminalTabs = computed(() => tabsStore.tabs)
+const showTerminalWorkspace = computed(() => route.path === '/terminal')
+const keepTerminalWorkspaceMounted = computed(() => showTerminalWorkspace.value || terminalTabs.value.length > 0)
 
 connectionsStore.fetchConnections().catch(() => {})
 
@@ -119,8 +122,13 @@ function handleTabClose(tabId: string, event: Event) {
       aria-hidden="true"
       @click="sidebarOpen = false"
     />
-    <main class="flex-1 overflow-auto min-w-0">
-      <RouterView />
-    </main>
-  </div>
-</template>
+    <main class="flex-1 overflow-auto min-w-0">
+      <div v-if="keepTerminalWorkspaceMounted" v-show="showTerminalWorkspace" class="h-full">
+        <TerminalWorkspaceView :visible="showTerminalWorkspace" />
+      </div>
+      <RouterView v-slot="{ Component }">
+        <component :is="Component" v-if="!showTerminalWorkspace" />
+      </RouterView>
+    </main>
+  </div>
+</template>