feat(backend): 添加 OAuth 能力查询接口，改进 OAuth 客户端选择逻辑

Handler 改进：
- 添加 GET /api/v1/admin/gemini/oauth/capabilities 接口
- 简化 GenerateAuthURL，redirect_uri 由服务层决定

Repository 改进：
- ExchangeCode/RefreshToken 根据 oauthType 选择正确的 OAuth 客户端
- Code Assist 始终使用内置客户端，AI Studio 使用用户配置的客户端

backend/internal/handler/admin/gemini_oauth_handler.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
@@ -19,6 +18,12 @@ func NewGeminiOAuthHandler(geminiOAuthService *service.GeminiOAuthService) *Gemi
 	return &GeminiOAuthHandler{geminiOAuthService: geminiOAuthService}
 }
 
+// GET /api/v1/admin/gemini/oauth/capabilities
+func (h *GeminiOAuthHandler) GetCapabilities(c *gin.Context) {
+	cfg := h.geminiOAuthService.GetOAuthConfig()
+	response.Success(c, cfg)
+}
+
 type GeminiGenerateAuthURLRequest struct {
 	ProxyID   *int64 `json:"proxy_id"`
 	ProjectID string `json:"project_id"`
@@ -46,12 +51,9 @@ func (h *GeminiOAuthHandler) GenerateAuthURL(c *gin.Context) {
 		return
 	}
 
+	// Always pass the "hosted" callback URI; the OAuth service may override it depending on
+	// oauth_type and whether the built-in Gemini CLI OAuth client is used.
 	redirectURI := deriveGeminiRedirectURI(c)
-	if oauthType == "ai_studio" {
-		// AI Studio OAuth uses a localhost redirect URI to support the "copy/paste callback URL"
-		// flow (no server-side callback endpoint needed).
-		redirectURI = geminicli.AIStudioOAuthRedirectURI
-	}
 	result, err := h.geminiOAuthService.GenerateAuthURL(c.Request.Context(), req.ProxyID, redirectURI, req.ProjectID, oauthType)
 	if err != nil {
 		msg := err.Error()