优化：工具栏高度对齐+面板统一+远程连接架构+自动恢复预览

- 工具栏：面包屑与右侧组件像素级等高(:deep 34px)、合并重复search handler、统一分隔符样式、删除死代码
- 面板对齐：三面板header统一padding/font-size、文件列表分页固定底部(自定义紧凑)、表头默认隐藏、滚动条统一样式
- 预览区：始终显示空白预览面板、重启自动恢复上次打开文件
- 收藏夹：简化计数显示(共N项)
- 远程连接：ConnectionIndicator自适应UI(无远程显示mini云图标)、ConnectionDialog支持编辑配置、transport抽象层(本地Wails/远程HTTP双模式)、agent后端模块

internal/agent/model/response.go

@@ -0,0 +1,41 @@
+package model
+
+import "net/http"
+
+type Response struct {
+	Code    int         `json:"code"`
+	Data    interface{} `json:"data,omitempty"`
+	Message string      `json:"message,omitempty"`
+}
+
+func OK(data interface{}) Response {
+	return Response{Code: http.StatusOK, Data: data}
+}
+
+func Created(data interface{}) Response {
+	return Response{Code: http.StatusCreated, Data: data}
+}
+
+func NoContent() Response {
+	return Response{Code: http.StatusNoContent}
+}
+
+func BadRequest(msg string) Response {
+	return Response{Code: http.StatusBadRequest, Message: msg}
+}
+
+func Unauthorized(msg string) Response {
+	return Response{Code: http.StatusUnauthorized, Message: msg}
+}
+
+func Forbidden(msg string) Response {
+	return Response{Code: http.StatusForbidden, Message: msg}
+}
+
+func NotFound(msg string) Response {
+	return Response{Code: http.StatusNotFound, Message: msg}
+}
+
+func InternalError(msg string) Response {
+	return Response{Code: http.StatusInternalServerError, Message: msg}
+}