feat(api): add virtual_path field on fs/list and fs/get responses

The existing `path` field on ObjResp/ObjLabelResp returns whatever the
storage driver writes into model.Object.Path, and that contract has
quietly diverged across drivers:

- Local sets it to the physical disk path
  (e.g. `/data/data/com.termux/files/home/storage/download/foo.tar.gz`)
- Cloud drivers (Quark / Baidu / 115 / …) leave it empty
- Some other drivers fill it with their own internal id-like path

Clients that need the canonical alist virtual path (e.g. for `share`,
`fs/copy`, navigation) cannot rely on `path` alone. The frontend has
been working around this with branchy code such as
`pathJoin(getCurrentPath(), name)` in some places and
`obj.path` in others; one such mismatch caused a share regression on
Local mounts whose root_folder_path differs from the mount path
(visible as `failed get storage: storage not found; rawPath: ...`).

Add a `virtual_path` field that is always the alist virtual path for
the object:

- `toObjsResp`  -> `FixAndCleanPath(stdpath.Join(parent, obj.Name))`
- `FsGet`       -> `FixAndCleanPath(reqPath)`

`path` is left untouched for backwards compatibility. Clients should
prefer `virtual_path` for anything that talks to other alist APIs.

server/handles/fsread.go

@@ -35,6 +35,7 @@ type DirReq struct {
 type ObjResp struct {
 	Id           string                     `json:"id"`
 	Path         string                     `json:"path"`
+	VirtualPath  string                     `json:"virtual_path"`
 	Name         string                     `json:"name"`
 	Size         int64                      `json:"size"`
 	IsDir        bool                       `json:"is_dir"`
@@ -65,6 +66,7 @@ type FsListResp struct {
 type ObjLabelResp struct {
 	Id           string                     `json:"id"`
 	Path         string                     `json:"path"`
+	VirtualPath  string                     `json:"virtual_path"`
 	Name         string                     `json:"name"`
 	Size         int64                      `json:"size"`
 	IsDir        bool                       `json:"is_dir"`
@@ -318,6 +320,7 @@ func toObjsResp(objs []model.Obj, parent string, encrypt bool) []ObjLabelResp {
 		resp = append(resp, ObjLabelResp{
 			Id:           obj.GetID(),
 			Path:         obj.GetPath(),
+			VirtualPath:  utils.FixAndCleanPath(stdpath.Join(parent, obj.GetName())),
 			Name:         obj.GetName(),
 			Size:         obj.GetSize(),
 			IsDir:        obj.IsDir(),
@@ -452,6 +455,7 @@ func FsGet(c *gin.Context) {
 		ObjResp: ObjResp{
 			Id:           obj.GetID(),
 			Path:         obj.GetPath(),
+			VirtualPath:  utils.FixAndCleanPath(reqPath),
 			Name:         obj.GetName(),
 			Size:         obj.GetSize(),
 			IsDir:        obj.IsDir(),