feat: full-field display for message/staff/accounts (raw_json + drawers)

Replicates the Customer all-fields pattern across the remaining three
entities. Backend now exposes `raw_json` on MessageOut / StaffOut /
AccountOut so the frontend can render every 云客 source field without a
second endpoint. Each entity gains a Chinese-label field utility + an
el-drawer surfacing all fields (structured-first, raw_json fallback,
unknown keys under 其他).

Backend:
- schemas.py: add `raw_json: dict | None` to MessageOut / StaffOut /
  AccountOut. MessageOut/StaffOut pick the column up via from_attributes;
  AccountOut is built explicitly in the router, which now threads
  `raw_json` through from both WechatAccount and WhatsappAccount rows.
- tests: new raw_json serialization tests for messages, staff, accounts
  (assert populated dict round-trips verbatim + NULL serializes as null).

Frontend (yunkes/):
- New utils: messageFields.ts, staffFields.ts, accountFields.ts — each
  with a 中文 label map, ordered groups, structured-column preference,
  and an 其他 fallback for unknown raw_json keys.
- Message: clicking a chat bubble opens an all-fields drawer (referMsg
  renders as pretty-printed JSON). Image preview + file download links
  stopPropagation so their own interactions still work.
- Staff: row click → drawer; columns relabeled (员工ID, 销售手机).
- Accounts: new AccountList.vue at /accounts with a 账号 nav item,
  unified WeChat/WhatsApp table (渠道/账号ID/昵称/微信号/归属员工/好友数/
  群数/收发/在职/最后更新), channel filter, pagination, row-click drawer.
- api: add Account type + getAccounts(); add raw_json to Message/Staff.

Co-Authored-By: Claude <noreply@anthropic.com>

yunkes/src/layouts/MainLayout.vue

@@ -123,6 +123,10 @@ onMounted(async () => {
           <el-icon><Avatar /></el-icon>
           <span>员工</span>
         </el-menu-item>
+        <el-menu-item index="/accounts">
+          <el-icon><Iphone /></el-icon>
+          <span>账号</span>
+        </el-menu-item>
         <el-menu-item index="/groups">
           <el-icon><User /></el-icon>
           <span>群</span>