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/components/TerminalWidget.vue

@@ -1,14 +1,15 @@
-<script setup lang="ts">
-import { ref, onMounted, onUnmounted } from 'vue'
-import { Terminal } from 'xterm'
-import { AttachAddon } from '@xterm/addon-attach'
-import { FitAddon } from '@xterm/addon-fit'
-import { useAuthStore } from '../stores/auth'
-import 'xterm/css/xterm.css'
-
-const props = defineProps<{
-  connectionId: number
-}>()
+<script setup lang="ts">
+import { nextTick, ref, onMounted, onUnmounted, watch } from 'vue'
+import { Terminal } from 'xterm'
+import { AttachAddon } from '@xterm/addon-attach'
+import { FitAddon } from '@xterm/addon-fit'
+import { useAuthStore } from '../stores/auth'
+import 'xterm/css/xterm.css'
+
+const props = defineProps<{
+  connectionId: number
+  active?: boolean
+}>()
 
 const containerRef = ref<HTMLElement | null>(null)
 const status = ref<'connecting' | 'connected' | 'error'>('connecting')
@@ -16,8 +17,23 @@ const errorMessage = ref('')
 
 let term: Terminal | null = null
 let fitAddon: FitAddon | null = null
-let ws: WebSocket | null = null
-let resizeObserver: ResizeObserver | null = null
+let ws: WebSocket | null = null
+let resizeObserver: ResizeObserver | null = null
+
+function fitTerminal() {
+  fitAddon?.fit()
+}
+
+function focusTerminal() {
+  term?.focus()
+}
+
+function refreshTerminalLayout() {
+  nextTick(() => {
+    fitTerminal()
+    focusTerminal()
+  })
+}
 
 function getWsUrl(): string {
   const authStore = useAuthStore()
@@ -71,8 +87,8 @@ onMounted(() => {
 
   fitAddon = new FitAddon()
   term.loadAddon(fitAddon)
-  term.open(containerRef.value)
-  fitAddon.fit()
+  term.open(containerRef.value)
+  fitTerminal()
 
   try {
     ws = new WebSocket(getWsUrl())
@@ -83,11 +99,11 @@ onMounted(() => {
   }
 
   ws.onopen = () => {
-    status.value = 'connected'
-    const attachAddon = new AttachAddon(ws!)
-    term!.loadAddon(attachAddon)
-    fitAddon?.fit()
-  }
+    status.value = 'connected'
+    const attachAddon = new AttachAddon(ws!)
+    term!.loadAddon(attachAddon)
+    refreshTerminalLayout()
+  }
 
   ws.onerror = () => {
     if (status.value === 'connecting') {
@@ -102,11 +118,21 @@ onMounted(() => {
     }
   }
 
-  resizeObserver = new ResizeObserver(() => {
-    fitAddon?.fit()
-  })
-  resizeObserver.observe(containerRef.value)
-})
+  resizeObserver = new ResizeObserver(() => {
+    fitTerminal()
+  })
+  resizeObserver.observe(containerRef.value)
+
+  if (props.active !== false) {
+    refreshTerminalLayout()
+  }
+})
+
+watch(() => props.active, (active) => {
+  if (active) {
+    refreshTerminalLayout()
+  }
+})
 
 onUnmounted(() => {
   cleanup()