fix(electron): fix data migration path mismatch after package extraction

The extraction of Electron code to apps/desktop/ changed the package name
from "ChatLab" to "@openchatlab/desktop", which caused app.getPath('userData')
to return a wrong path in dev mode. This made the migration system unable to
find existing databases, resulting in a fresh-app experience for upgrading users.

- Add productName "ChatLab" to desktop package.json to ensure correct userData path
- Add verifyDataPath() safety net: auto-correct config.toml when configured dir
  has no databases but old Electron path does

apps/desktop/package.json

@@ -1,6 +1,7 @@
 {
   "name": "@openchatlab/desktop",
   "version": "0.19.0",
+  "productName": "ChatLab",
   "private": true,
   "description": "ChatLab Desktop — Electron 桌面客户端",
   "main": "./out/main/index.js",