feat: hover preview for truncated JSON content

New HoverPreview component shows a floating tooltip with full
content on mouse hover (400ms delay). Applied to:
- MessageStream: collapsed + expanded payload views
- PresetList: payload preview in table
- LogPanel: details section

The popup follows the mouse cursor, auto-positions within viewport,
and supports monospace code formatting with scroll for long content.

src/components/common/HoverPreview.vue

@@ -0,0 +1,119 @@
+<script setup lang="ts">
+import { ref, onMounted, onUnmounted } from 'vue'
+
+const props = withDefaults(defineProps<{
+  /** Full text to show in the preview */
+  text: string
+  /** Maximum width of the preview popup */
+  maxWidth?: number
+  /** Maximum height of the preview popup */
+  maxHeight?: number
+  /** Delay before showing (ms) */
+  delay?: number
+}>(), {
+  maxWidth: 480,
+  maxHeight: 360,
+  delay: 400,
+})
+
+const visible = ref(false)
+const x = ref(0)
+const y = ref(0)
+let timer: ReturnType<typeof setTimeout> | null = null
+const wrapper = ref<HTMLElement | null>(null)
+
+function onMouseEnter(ev: MouseEvent) {
+  timer = setTimeout(() => {
+    visible.value = true
+    updatePosition(ev)
+  }, props.delay)
+}
+
+function onMouseMove(ev: MouseEvent) {
+  if (visible.value) {
+    updatePosition(ev)
+  }
+}
+
+function onMouseLeave() {
+  if (timer) {
+    clearTimeout(timer)
+    timer = null
+  }
+  visible.value = false
+}
+
+function updatePosition(ev: MouseEvent) {
+  const pad = 12
+  let left = ev.clientX + pad
+  let top = ev.clientY + pad
+
+  // Keep within viewport
+  const预估Width = Math.min(props.maxWidth, 480)
+  if (left + 预估Width > window.innerWidth - 16) {
+    left = ev.clientX - 预估Width - pad
+  }
+  if (top + 360 > window.innerHeight - 16) {
+    top = window.innerHeight - 360 - 16
+  }
+  if (left < 8) left = 8
+  if (top < 8) top = 8
+
+  x.value = left
+  y.value = top
+}
+</script>
+
+<template>
+  <span
+    ref="wrapper"
+    class="hover-preview-wrap"
+    @mouseenter="onMouseEnter"
+    @mousemove="onMouseMove"
+    @mouseleave="onMouseLeave"
+  >
+    <slot />
+    <Teleport to="body">
+      <div
+        v-if="visible && text"
+        class="hover-preview-popup"
+        :style="{
+          left: x + 'px',
+          top: y + 'px',
+          maxWidth: maxWidth + 'px',
+          maxHeight: maxHeight + 'px',
+        }"
+      >
+        <pre class="hover-preview-content"><code>{{ text }}</code></pre>
+      </div>
+    </Teleport>
+  </span>
+</template>
+
+<style scoped>
+.hover-preview-wrap {
+  display: inline;
+  cursor: pointer;
+  position: relative;
+}
+.hover-preview-popup {
+  position: fixed;
+  z-index: 10000;
+  background: var(--comfort-bg-card, var(--el-bg-color-overlay));
+  border: 1px solid var(--comfort-border, var(--el-border-color));
+  border-radius: 6px;
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.12), 0 1px 4px rgba(0, 0, 0, 0.08);
+  padding: 8px 10px;
+  overflow: auto;
+  pointer-events: none;
+}
+.hover-preview-content {
+  margin: 0;
+  font-family: var(--comfort-font-mono, 'Cascadia Code', 'Fira Code', 'Consolas', monospace);
+  font-size: 12px;
+  line-height: 1.5;
+  color: var(--comfort-text, var(--el-text-color-regular));
+  white-space: pre-wrap;
+  word-break: break-all;
+}
+</style>

src/components/dashboard/LogPanel.vue

@@ -3,6 +3,7 @@ import { ref, computed, watch, nextTick } from 'vue'
 import { useLogs } from '../../stores/useLogs'
 import { mqttGetLogDir } from '../../stores/useMqttBridge'
 import { prettyJson } from '../../utils/format'
+import HoverPreview from '../common/HoverPreview.vue'
 import { ElMessage } from 'element-plus'
 
 const { logs, clear } = useLogs()
@@ -122,7 +123,7 @@ function eventIcon(event: string): string {
           <span v-if="log.details" class="log-dtl-btn" @click="toggleDetails(log.id)">{{ expandedDetails.has(log.id) ? '收起' : '详情' }}</span>
         </div>
         <div v-if="log.details && expandedDetails.has(log.id)" class="log-dtl-content">
-          {{ prettyJson(log.details) }}
+          <HoverPreview :text="prettyJson(log.details)">{{ prettyJson(log.details) }}</HoverPreview>
         </div>
       </div>
     </div>

src/components/dashboard/MessageStream.vue

@@ -2,6 +2,7 @@
 import { ref, watch, nextTick } from 'vue'
 import type { MqttMessage } from '../../types/mqtt'
 import { prettyJson, isJson, truncate } from '../../utils/format'
+import HoverPreview from '../common/HoverPreview.vue'
 
 const props = defineProps<{
   messages: MqttMessage[]
@@ -168,8 +169,10 @@ function formatTime(ts: number): string {
         <pre
           v-if="!collapsedPayloads.has(msg.id)"
           :class="['payload-full', { 'is-json': getEffectiveFormat(msg) === 'json' }]"
-        >{{ formatPayload(msg) }}</pre>
-        <pre v-else class="payload-truncated">{{ truncate(prettyJson(msg.payload), 200) }}</pre>
+        ><HoverPreview :text="formatPayload(msg)">{{ formatPayload(msg) }}</HoverPreview></pre>
+        <pre v-else class="payload-truncated">
+              <HoverPreview :text="prettyJson(msg.payload)">{{ truncate(prettyJson(msg.payload), 200) }}</HoverPreview>
+            </pre>
       </div>
     </div>
   </div>

src/components/presets/PresetList.vue

@@ -4,6 +4,7 @@ import { ElMessage, ElMessageBox } from 'element-plus'
 import type { Preset } from '../../types/mqtt'
 import { usePresets } from '../../stores/usePresets'
 import { prettyJson, truncate } from '../../utils/format'
+import HoverPreview from '../common/HoverPreview.vue'
 import PresetForm from './PresetForm.vue'
 
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
@@ -77,7 +78,9 @@ const qosLabel = (qos: number) => `QoS ${qos}`
       </el-table-column>
       <el-table-column label="Payload 预览" min-width="200">
         <template #default="{ row }">
-          <span class="payload-preview">{{ truncate(prettyJson(row.payloadTemplate), 50) }}</span>
+          <span class="payload-preview">
+            <HoverPreview :text="prettyJson(row.payloadTemplate)">{{ truncate(prettyJson(row.payloadTemplate), 50) }}</HoverPreview>
+          </span>
         </template>
       </el-table-column>
       <el-table-column label="操作" width="160" fixed="right">