fix: complete vault layout transition — fs_path everywhere, no more spaces/

- notes.Create(): .md files stored in parent node's fs_path folder
- files.CopyIntoVault/CreateEmptyFile/Duplicate: use parent fs_path
- files.AddPathCopy/AddPathLink: use parent fs_path, set folder fs_path
- files.DeleteNodeAndChildren: move physical folder to .verstak/trash
- UpdateFsPathRecursive: use SafeDisplayNameToPathSegment(child.Title)
- sync_apply.go note ops: use fs_path instead of spaces/
- internal/gui/server.go file upload: use n.FsPath instead of nodeSlug
- VaultCheck diagnostic: walk nodes/files, verify paths on disk
- Tests: create/rename/move/delete/name-conflict/vault-check all pass

internal/gui/server.go

@@ -309,14 +309,18 @@ func (s *Server) handleFileUpload(w http.ResponseWriter, r *http.Request) {
 	var req struct {
 		NodeID   string `json:"node_id"`
 		FilePath string `json:"file_path"`
-		NodeSlug string `json:"node_slug"`
 	}
 	json.NewDecoder(r.Body).Decode(&req)
 	if req.NodeID == "" || req.FilePath == "" {
 		jsonErr(w, 400, "node_id and file_path required")
 		return
 	}
-	rec, err := s.files.CopyIntoVault(req.NodeID, req.FilePath, req.NodeSlug)
+	n, err := s.nodes.GetActive(req.NodeID)
+	if err != nil {
+		jsonErr(w, 404, "node not found")
+		return
+	}
+	rec, err := s.files.CopyIntoVault(req.NodeID, req.FilePath, n.FsPath)
 	if err != nil {
 		jsonErr(w, 500, err.Error())
 		return