feat(import): update demo data sources to 4 files

Switch from 2 demo files (group + private) to 4 files:
- demo-group.json
- demo-private-A-cuilan.json
- demo-private-B-wukong.json
- demo-private-C-spider.json

Update DemoImportResult.privateSessionId to privateSessionIds array
to support multiple private chat demos.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/desktop/preload/apis/chat.ts

@@ -501,7 +501,7 @@ export const chatApi = {
   ): Promise<{
     success: boolean
     groupSessionId?: string
-    privateSessionId?: string
+    privateSessionIds?: string[]
     error?: string
   }> => {
     return ipcRenderer.invoke('demo:downloadAndImport', locale)