perf(frontend): 收敛导航预取并去重首屏请求

- 导航预取仅保留 pointerdown 触发，移除 mouseenter/focus，避免鼠标划过误触发
- 后台预取只做组件懒加载，不再预取各页业务数据，减少首屏资源争抢
- 版本状态检查增加 sessionStorage 缓存（正常 20 分钟 / 错误 5 分钟 TTL）
- fetchModules、必读公告拉取增加请求去重，避免并发重复请求
- 更新检查改用可清理的定时器，组件卸载时清理
- UsageRecordsTable 搜索防抖改为自定义实现，卸载时取消挂起 emit 并补充测试

frontend/src/features/usage/components/__tests__/UsageRecordsTable.spec.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
-import { createApp, defineComponent, h, type App } from 'vue'
+import { createApp, defineComponent, h, nextTick, type App } from 'vue'
 import UsageRecordsTable from '../UsageRecordsTable.vue'
 import type { UsageRecord } from '../../types'
 
@@ -167,6 +167,7 @@ afterEach(() => {
     app.unmount()
     root.remove()
   }
+  vi.useRealTimers()
 })
 
 describe('UsageRecordsTable', () => {
@@ -314,6 +315,31 @@ describe('UsageRecordsTable', () => {
     expect(onUpdateHideUnknownRecords).toHaveBeenCalledWith(true)
   })
 
+  it('debounces usage search updates', async () => {
+    vi.useFakeTimers()
+    const onUpdateFilterSearch = vi.fn()
+    const root = mountUsageRecordsTable([buildRecord()], {
+      'onUpdate:filterSearch': onUpdateFilterSearch,
+    })
+    const input = root.querySelector<HTMLInputElement>('#usage-records-search')
+    expect(input).not.toBeNull()
+
+    input!.value = 'a'
+    input!.dispatchEvent(new Event('input'))
+    input!.value = 'ab'
+    input!.dispatchEvent(new Event('input'))
+    input!.value = 'abc'
+    input!.dispatchEvent(new Event('input'))
+    await nextTick()
+
+    await vi.advanceTimersByTimeAsync(299)
+    expect(onUpdateFilterSearch).not.toHaveBeenCalled()
+
+    await vi.advanceTimersByTimeAsync(1)
+    expect(onUpdateFilterSearch).toHaveBeenCalledTimes(1)
+    expect(onUpdateFilterSearch).toHaveBeenCalledWith('abc')
+  })
+
   it('shows retry and fallback markers together when both flags are set', () => {
     const root = mountUsageRecordsTable([buildRecord({
       has_fallback: true,