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/account-discovery.test.ts

@@ -0,0 +1,69 @@
+import fs from 'fs-extra'
+import os from 'os'
+import path from 'path'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+const mocked = vi.hoisted(() => ({
+  userData: `${process.env.TEMP || process.env.TMP || '.'}/wxe-account-discovery-tests`
+}))
+
+vi.mock('electron', () => ({
+  app: { getPath: () => mocked.userData }
+}))
+
+import { discoverAccounts } from '../../src/main/services/account-discovery'
+
+describe('account discovery', () => {
+  let root: string
+  const keyStore = {
+    getStatus: vi.fn(async (accountRoot: string) => ({
+      saved: accountRoot.endsWith('account-b'),
+      encryptionAvailable: true
+    }))
+  }
+
+  beforeEach(async () => {
+    root = await fs.mkdtemp(path.join(os.tmpdir(), 'wxe-accounts-'))
+    await Promise.all(
+      ['account-a', 'account-b', 'account-c'].map((name) =>
+        fs.ensureDir(path.join(root, name, 'db_storage'))
+      )
+    )
+    await fs.ensureDir(path.join(root, 'Backup'))
+  })
+
+  afterEach(async () => {
+    await fs.remove(root)
+    await fs.remove(mocked.userData)
+  })
+
+  it('rejects an invalid Backup directory without continuing', async () => {
+    const result = await discoverAccounts(path.join(root, 'Backup'), keyStore as never)
+    expect(result.success).toBe(false)
+    expect(result.accounts).toEqual([])
+  })
+
+  it('lists every direct account and never preselects one from a root directory', async () => {
+    const result = await discoverAccounts(root, keyStore as never)
+    expect(result.success).toBe(true)
+    expect(result.accounts.map((account) => account.directoryName).sort()).toEqual([
+      'account-a',
+      'account-b',
+      'account-c'
+    ])
+    expect(result.preselectedAccountId).toBeUndefined()
+    expect(
+      result.accounts.find((account) => account.directoryName === 'account-b')?.hasSavedDbKey
+    ).toBe(true)
+  })
+
+  it('preselects a directly selected account directory while retaining its card', async () => {
+    const accountRoot = path.join(root, 'account-c')
+    const result = await discoverAccounts(accountRoot, keyStore as never)
+    expect(result.success).toBe(true)
+    expect(result.accounts).toHaveLength(1)
+    expect(result.accounts[0].accountRoot).toBe(accountRoot)
+    expect(result.preselectedAccountId).toBe(result.accounts[0].id)
+    expect(result.accounts[0].selectedByInput).toBe(true)
+  })
+})