fix(auth): clear cached silentLoginPromise on logout to allow re-authentication

After logout, the cached silentLoginPromise caused silentLogin() to return
a stale result instead of re-executing loginByComputerName(), leaving
sessionManager.currentUser as null and making subsequent switchUser() calls fail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/services/auth/auth-application-service.ts

@@ -215,6 +215,7 @@ export class AuthApplicationService {
         }
 
         this.sessionManager.logout()
+        this.silentLoginPromise = null
         await this.updateService.setUserContext(null)
       },
       { operation: 'logout' }