feat: 完善多账号连接诊断与聊天媒体导出

- 新增微信账号发现、环境诊断和分步数据库连接引导
- 支持按账号安全保存数据库密钥及快速切换账号
- 完善 WCDB 历史消息分片读取和分页状态提示
- 支持导出图片、视频和语音，提供原图优先及缩略图回退
- 更新安装指引、兼容版本说明和相关自动化测试

tests/unit/wcdb-message-shards.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, it, vi } from 'vitest'
+import { Wcdb4Client, type Wcdb4Message } from '../../src/main/wcdb4-client'
+
+const message = (id: string, year: number): Wcdb4Message => ({
+  mesLocalID: id,
+  serverId: `server-${id}`,
+  mesDes: 0,
+  messageType: '1',
+  msgCreateTime: String(Math.floor(Date.UTC(year, 0, 1) / 1000)),
+  msgContent: `fixture-${year}`,
+  raw: {}
+})
+
+describe('WCDB message shard pagination', () => {
+  it('merges cursor and all-store rows for a bounded cross-year page', async () => {
+    const cursor = vi.fn(async () => [message('2025', 2025)])
+    const tableScan = vi.fn(async () => [message('2017', 2017), message('2025', 2025)])
+    const client = Object.assign(Object.create(Wcdb4Client.prototype), {
+      wcdbGetMessageTableStats: vi.fn(),
+      wcdbExecQuery: vi.fn(),
+      getMessagesByCursorAsync: cursor,
+      getMessagesByTableScanAsync: tableScan
+    }) as Wcdb4Client
+
+    const result = await client.getMessagesAsync(
+      'fixture@chatroom',
+      undefined,
+      Math.floor(Date.UTC(2026, 0, 1) / 1000),
+      { limit: 20 }
+    )
+
+    expect(tableScan).toHaveBeenCalledOnce()
+    expect(result.map((item) => item.msgContent)).toEqual(['fixture-2017', 'fixture-2025'])
+  })
+
+  it('reports an unsupported shard query instead of claiming history ended', async () => {
+    const client = Object.assign(Object.create(Wcdb4Client.prototype), {
+      getMessagesByCursorAsync: vi.fn(async () => [])
+    }) as Wcdb4Client
+
+    await expect(
+      client.getMessagesAsync('fixture@chatroom', undefined, 1_767_225_600, { limit: 20 })
+    ).rejects.toThrow('无法检查历史消息分片')
+  })
+})