fix(desktop): align conversationService log format + document deferred import

Code review follow-up on 94b445c:
- console.error 改用 err instanceof Error ? err.message : err (跟 hahaOAuthService 一致)
- dynamic import hahaOAuthService 加注释说明 defer 目的

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/server/services/conversationService.ts

@@ -528,13 +528,18 @@ export class ConversationService {
       CLAUDE_CODE_ENTRYPOINT: 'claude-desktop',
     }
     try {
+      // deferred import: avoids instantiating the OAuth singleton on every
+      // ConversationService construction — only loaded when official mode hits.
       const { hahaOAuthService } = await import('./hahaOAuthService.js')
       const token = await hahaOAuthService.ensureFreshAccessToken()
       if (token) {
         env.CLAUDE_CODE_OAUTH_TOKEN = token
       }
     } catch (err) {
-      console.error('[conversationService] ensureFreshAccessToken failed:', err)
+      console.error(
+        '[conversationService] ensureFreshAccessToken failed:',
+        err instanceof Error ? err.message : err,
+      )
     }
     return env
   }