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/handler/system_handler.go

@@ -0,0 +1,113 @@
+package handler
+
+import (
+	"net/http"
+	"os"
+	"runtime"
+	"strings"
+
+	"u-desk/internal/agent/model"
+
+	"github.com/labstack/echo/v4"
+)
+
+// Ping 健康检查
+func (h *Handler) Ping(c echo.Context) error {
+	return c.JSON(http.StatusOK, model.OK(map[string]string{
+		"status": "ok",
+	}))
+}
+
+// Info 返回 Agent 信息
+func (h *Handler) Info(c echo.Context) error {
+	hostname, _ := os.Hostname()
+	return c.JSON(http.StatusOK, model.OK(map[string]interface{}{
+		"version":  "0.1.0",
+		"os":       runtime.GOOS,
+		"arch":     runtime.GOARCH,
+		"hostname": hostname,
+	}))
+}
+
+// CommonPaths 返回常用系统路径
+func (h *Handler) CommonPaths(c echo.Context) error {
+	paths := map[string]string{}
+
+	home, _ := os.UserHomeDir()
+	if home != "" {
+		paths["home"] = home
+		paths["desktop"] = home + "/Desktop"
+		paths["documents"] = home + "/Documents"
+		paths["downloads"] = home + "/Downloads"
+	}
+
+	// 根据平台添加盘符/根路径
+	if runtime.GOOS == "windows" {
+		for _, drive := range "ABCDEFGHIJKLMNOPQRSTUVWXYZ" {
+			_, err := os.Stat(string(drive) + ":\\")
+			if err == nil {
+				paths["drive_"+strings.ToLower(string(drive))] = string(drive) + ":\\"
+			}
+		}
+	} else {
+		paths["root"] = "/"
+		_, err := os.Stat("/home")
+		if err == nil {
+			paths["users"] = "/home"
+		}
+	}
+
+	return c.JSON(http.StatusOK, model.OK(paths))
+}
+
+// Drives 返回可用磁盘列表
+func (h *Handler) Drives(c echo.Context) error {
+	type DriveInfo struct {
+		Name    string `json:"name"`
+		Path    string `json:"path"`
+		FsType  string `json:"fs_type,omitempty"`
+		Total   uint64 `json:"total"`
+		Free    uint64 `json:"free"`
+	}
+
+	var drives []DriveInfo
+
+	if runtime.GOOS == "windows" {
+		for _, drive := range "ABCDEFGHIJKLMNOPQRSTUVWXYZ" {
+			drivePath := string(drive) + ":\\"
+			if _, err := os.Stat(drivePath); err != nil {
+				continue
+			}
+			drives = append(drives, DriveInfo{
+				Name: strings.ToLower(string(drive)),
+				Path: drivePath,
+				Total: 0,
+				Free:  0,
+			})
+		}
+	} else {
+		parts, err := os.ReadDir("/")
+		if err == nil {
+			for _, p := range parts {
+				name := p.Name()
+				if len(name) == 2 && name[0] != '.' && name[1] != '.' && p.IsDir() {
+					// 可能是挂载点
+					fullPath := "/" + name
+					if stat, err := os.Stat(fullPath); err == nil && stat.IsDir() {
+						drives = append(drives, DriveInfo{
+							Name: name,
+							Path: fullPath,
+						})
+						_ = stat
+					}
+				}
+			}
+		}
+		// 至少返回根目录
+		if len(drives) == 0 {
+			drives = append(drives, DriveInfo{Name: "/", Path: "/"})
+		}
+	}
+
+	return c.JSON(http.StatusOK, model.OK(drives))
+}