feat: 新增群员统计与退群事件，并把本地索引集中到设置页

- 群员统计：档案中按群统计发言排行与未发言成员，可复制纯文本、按平台能力发送
- 退群事件永久保存，并以本地推断条目并入档案消息流
- 设置新增本地索引页，集中管理聊天记录索引与图片文字索引

tests/unit/group-exit-event-message.test.ts

@@ -0,0 +1,120 @@
+import { describe, expect, it } from 'vitest'
+import {
+  GROUP_EXIT_EVENT_MESSAGE_TYPE,
+  isGroupExitEventMessage,
+  mergeGroupExitEvents,
+  toGroupExitEventMessage
+} from '../../src/shared/group-exit-event-message'
+import type { GroupExitMonitorEvent } from '../../src/shared/group-exit-monitor'
+import type { Message } from '../../src/shared/types'
+
+const at = (year: number, month: number, day: number, hour = 0, minute = 0): number =>
+  new Date(year, month - 1, day, hour, minute).getTime()
+
+const makeEvent = (overrides: Partial<GroupExitMonitorEvent> = {}): GroupExitMonitorEvent => ({
+  id: 'evt-1',
+  contactId: 'g'.repeat(32),
+  roomId: 'room@chatroom',
+  groupName: '研发群',
+  memberWxid: 'wxid_a',
+  memberName: '老张',
+  previousCount: 10,
+  currentCount: 9,
+  delta: -1,
+  message: '老张退出了研发群',
+  detectedAt: at(2026, 9, 20, 14, 30),
+  ...overrides
+})
+
+const makeMessage = (id: string, createTimeSec: number): Message => ({
+  id,
+  from: 'user',
+  type: '文本',
+  datetime: '2026-09-20 14:00',
+  content: 'hello',
+  isSender: false,
+  createTime: createTimeSec
+})
+
+describe('toGroupExitEventMessage', () => {
+  it('把事件映射成系统分支的消息，并带专用 type 标记', () => {
+    const message = toGroupExitEventMessage(makeEvent())
+
+    expect(message.id).toBe('group-exit:evt-1')
+    expect(message.from).toBe('system')
+    expect(message.type).toBe(GROUP_EXIT_EVENT_MESSAGE_TYPE)
+    expect(message.content).toBe('老张退出了研发群')
+    expect(message.isSender).toBe(false)
+    expect(message.senderId).toBe('wxid_a')
+    expect(message.sessionId).toBe('room@chatroom')
+    expect(isGroupExitEventMessage(message)).toBe(true)
+  })
+
+  it('⚠️ createTime 必须从毫秒换算成秒（全项目 Message.createTime 都是秒）', () => {
+    const detectedAt = at(2026, 9, 20, 14, 30)
+    const message = toGroupExitEventMessage(makeEvent({ detectedAt }))
+
+    // 直接塞毫秒会让事件被排到几万年之后，或者从时间轴上消失。
+    expect(message.createTime).toBe(Math.floor(detectedAt / 1000))
+    expect(message.createTime).toBeLessThan(1e11)
+  })
+
+  it('真实微信系统消息不会被误判为退群事件', () => {
+    const systemMessage: Message = {
+      id: 'm1',
+      from: 'system',
+      type: '系统消息',
+      datetime: '2026-09-20 14:00',
+      content: '某某加入了群聊',
+      isSender: false
+    }
+
+    expect(isGroupExitEventMessage(systemMessage)).toBe(false)
+  })
+})
+
+describe('mergeGroupExitEvents', () => {
+  it('按时间升序并入，事件落在正确位置', () => {
+    const messages = [
+      makeMessage('m1', Math.floor(at(2026, 9, 20, 10) / 1000)),
+      makeMessage('m2', Math.floor(at(2026, 9, 20, 16) / 1000))
+    ]
+    const events = [makeEvent({ detectedAt: at(2026, 9, 20, 14, 30) })]
+
+    const merged = mergeGroupExitEvents(messages, events)
+
+    expect(merged.map((message) => message.id)).toEqual(['m1', 'group-exit:evt-1', 'm2'])
+  })
+
+  it('重复合并是幂等的（广播会反复刷新，不能越插越多）', () => {
+    const messages = [makeMessage('m1', Math.floor(at(2026, 9, 20, 10) / 1000))]
+    const events = [makeEvent()]
+
+    const once = mergeGroupExitEvents(messages, events)
+    const twice = mergeGroupExitEvents(once, events)
+
+    expect(twice).toHaveLength(2)
+    expect(twice.map((message) => message.id)).toEqual(once.map((message) => message.id))
+  })
+
+  it('没有事件时原样返回同一个引用（避免无谓的重渲染）', () => {
+    const messages = [makeMessage('m1', 1_700_000_000)]
+    expect(mergeGroupExitEvents(messages, [])).toBe(messages)
+  })
+
+  it('多个事件各自成组、保持时间顺序', () => {
+    const messages = [makeMessage('m1', Math.floor(at(2026, 9, 20, 8) / 1000))]
+    const events = [
+      makeEvent({ id: 'e1', detectedAt: at(2026, 9, 20, 12) }),
+      makeEvent({ id: 'e2', detectedAt: at(2026, 9, 20, 15) })
+    ]
+
+    const merged = mergeGroupExitEvents(messages, events)
+
+    expect(merged.map((message) => message.id)).toEqual([
+      'm1',
+      'group-exit:e1',
+      'group-exit:e2'
+    ])
+  })
+})