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

src/main/services/account-discovery.ts

@@ -0,0 +1,58 @@
+import crypto from 'crypto'
+import fs from 'fs-extra'
+import path from 'path'
+import type { AccountDiscoveryResult, WechatAccountCandidate } from '../../shared/database-key'
+import { DatabaseKeyStore } from '../database-key-store'
+import { getBootstrapCache } from './bootstrap-cache'
+import { validateDbRoot } from './settings-store'
+
+function accountId(accountRoot: string): string {
+  return crypto.createHash('sha256').update(path.resolve(accountRoot).toLowerCase()).digest('hex')
+}
+
+export async function discoverAccounts(
+  inputPath: string,
+  keyStore: DatabaseKeyStore,
+  currentAccountRoot?: string
+): Promise<AccountDiscoveryResult> {
+  const validation = validateDbRoot(inputPath)
+  if (!validation.valid) return { success: false, accounts: [], error: validation.error }
+
+  const normalizedInput = path.resolve(inputPath)
+  const isAccount = await fs.pathExists(path.join(normalizedInput, 'db_storage'))
+  const roots = isAccount
+    ? [normalizedInput]
+    : (await fs.readdir(normalizedInput, { withFileTypes: true }))
+        .filter((entry) => entry.isDirectory())
+        .map((entry) => path.join(normalizedInput, entry.name))
+        .filter((candidate) => fs.existsSync(path.join(candidate, 'db_storage')))
+
+  const accounts: WechatAccountCandidate[] = await Promise.all(
+    roots.map(async (accountRoot) => {
+      const cached = getBootstrapCache(accountRoot)?.self
+      return {
+        id: accountId(accountRoot),
+        accountRoot,
+        directoryName: path.basename(accountRoot),
+        wxid: cached?.wxid,
+        nickname: cached?.nickname,
+        avatar: cached?.avatar,
+        hasSavedDbKey: (await keyStore.getStatus(accountRoot)).saved,
+        loginStatus: currentAccountRoot
+          ? path.resolve(currentAccountRoot).toLowerCase() ===
+            path.resolve(accountRoot).toLowerCase()
+            ? 'current'
+            : 'other'
+          : 'unknown',
+        selectedByInput: isAccount
+      }
+    })
+  )
+
+  return {
+    success: true,
+    inputKind: isAccount ? 'account' : 'root',
+    accounts,
+    preselectedAccountId: isAccount ? accounts[0]?.id : undefined
+  }
+}