feat(gemini): 优化 OAuth 和配额展示

主要改进：
- 修复 google_one OAuth scopes 配置问题
- 添加 Gemini 账号配额展示组件
- 优化 Code Assist 类型检测逻辑
- 添加 OAuth 测试用例

backend/internal/pkg/geminicli/constants.go

@@ -1,5 +1,3 @@
-// Package geminicli provides OAuth authentication and API client functionality
-// for Google's Gemini AI services, supporting both AI Studio and Code Assist endpoints.
 package geminicli
 
 import "time"
@@ -29,7 +27,9 @@ const (
 	DefaultAIStudioScopes = "https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/generative-language.retriever"
 
 	// DefaultScopes for Google One (personal Google accounts with Gemini access)
-	// Includes generative-language for Gemini API access and drive.readonly for storage tier detection
+	// Only used when a custom OAuth client is configured. When using the built-in Gemini CLI client,
+	// Google One uses DefaultCodeAssistScopes (same as code_assist) because the built-in client
+	// cannot request restricted scopes like generative-language.retriever or drive.readonly.
 	DefaultGoogleOneScopes = "https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/generative-language.retriever https://www.googleapis.com/auth/drive.readonly https://www.googleapis.com/auth/userinfo.email https://www.googleapis.com/auth/userinfo.profile"
 
 	// GeminiCLIRedirectURI is the redirect URI used by Gemini CLI for Code Assist OAuth.