feat(accounts): 自动刷新改为ETag增量同步并优化单账号更新体验

- 前端自动刷新改为 ETag/304 增量合并，减少全量重刷

- 单账号更新后增加静默窗口，避免刚更新即被自动刷新覆盖

- 列表筛选移除时改为待同步提示，不再立即触发全量补页

- 后端账号列表支持 If-None-Match，命中返回 304

- 单账号接口统一补充运行时容量字段并暴露 ETag 头

backend/internal/server/middleware/cors.go

@@ -70,6 +70,7 @@ func CORS(cfg config.CORSConfig) gin.HandlerFunc {
 			}
 			c.Writer.Header().Set("Access-Control-Allow-Headers", "Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token, Authorization, accept, origin, Cache-Control, X-Requested-With, X-API-Key")
 			c.Writer.Header().Set("Access-Control-Allow-Methods", "POST, OPTIONS, GET, PUT, DELETE, PATCH")
+			c.Writer.Header().Set("Access-Control-Expose-Headers", "ETag")
 			c.Writer.Header().Set("Access-Control-Max-Age", "86400")
 		}