feat(gemini): 完善 Gemini OAuth 配额系统和用量显示

主要改动：
- 后端：重构 Gemini 配额服务，支持多层级配额策略（GCP Standard/Free, Google One, AI Studio, Code Assist）
- 后端：优化 OAuth 服务，增强 tier_id 识别和存储逻辑
- 后端：改进用量统计服务，支持不同平台的配额查询
- 后端：优化限流服务，增加临时解除调度状态管理
- 前端：统一四种授权方式的用量显示格式和徽标样式
- 前端：增强账户配额信息展示，支持多种配额类型
- 前端：改进创建和重新授权模态框的用户体验
- 国际化：完善中英文配额相关文案
- 移除 CHANGELOG.md 文件

测试：所有单元测试通过

backend/internal/handler/admin/gemini_oauth_handler.go

@@ -30,6 +30,8 @@ type GeminiGenerateAuthURLRequest struct {
 	// OAuth 类型: "code_assist" (需要 project_id) 或 "ai_studio" (不需要 project_id)
 	// 默认为 "code_assist" 以保持向后兼容
 	OAuthType string `json:"oauth_type"`
+	// TierID is a user-selected tier to be used when auto detection is unavailable or fails.
+	TierID string `json:"tier_id"`
 }
 
 // GenerateAuthURL generates Google OAuth authorization URL for Gemini.
@@ -54,7 +56,7 @@ func (h *GeminiOAuthHandler) GenerateAuthURL(c *gin.Context) {
 	// 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)
-	result, err := h.geminiOAuthService.GenerateAuthURL(c.Request.Context(), req.ProxyID, redirectURI, req.ProjectID, oauthType)
+	result, err := h.geminiOAuthService.GenerateAuthURL(c.Request.Context(), req.ProxyID, redirectURI, req.ProjectID, oauthType, req.TierID)
 	if err != nil {
 		msg := err.Error()
 		// Treat missing/invalid OAuth client configuration as a user/config error.
@@ -76,6 +78,9 @@ type GeminiExchangeCodeRequest struct {
 	ProxyID   *int64 `json:"proxy_id"`
 	// OAuth 类型: "code_assist" 或 "ai_studio"，需要与 GenerateAuthURL 时的类型一致
 	OAuthType string `json:"oauth_type"`
+	// TierID is a user-selected tier to be used when auto detection is unavailable or fails.
+	// This field is optional; when omitted, the server uses the tier stored in the OAuth session.
+	TierID string `json:"tier_id"`
 }
 
 // ExchangeCode exchanges authorization code for tokens.
@@ -103,6 +108,7 @@ func (h *GeminiOAuthHandler) ExchangeCode(c *gin.Context) {
 		Code:      req.Code,
 		ProxyID:   req.ProxyID,
 		OAuthType: oauthType,
+		TierID:    req.TierID,
 	})
 	if err != nil {
 		response.BadRequest(c, "Failed to exchange code: "+err.Error())