Only show chevron in trees where appropriate

tree.go

@@ -12,6 +12,10 @@ import (
 type treeEntry struct {
 	Name string `json:"name"`
 	Kind string `json:"kind"`
+	// HasChildren reports whether a folder has any visible entries (under the
+	// same foldersOnly filter), so the client can hide the expand chevron on
+	// empty folders without a round-trip. Always false for files.
+	HasChildren bool `json:"hasChildren"`
 	// Children is populated only along the expandTo chain (see handleTree);
 	// omitted otherwise so the flat picker listing keeps its original shape.
 	Children []treeEntry `json:"children,omitempty"`
@@ -81,7 +85,11 @@ func listTreeEntries(fsPath string, foldersOnly bool) ([]treeEntry, error) {
 			continue
 		}
 		if e.IsDir() {
-			folders = append(folders, treeEntry{Name: name, Kind: "folder"})
+			folders = append(folders, treeEntry{
+				Name:        name,
+				Kind:        "folder",
+				HasChildren: folderHasChildren(filepath.Join(fsPath, name), foldersOnly),
+			})
 		} else {
 			if foldersOnly || name == "index.md" {
 				continue
@@ -94,6 +102,33 @@ func listTreeEntries(fsPath string, foldersOnly bool) ([]treeEntry, error) {
 	return append(folders, files...), nil
 }
 
+// folderHasChildren reports whether fsPath contains at least one entry that
+// listTreeEntries would list. It reads the directory in small batches and
+// stops at the first match, so large folders stay cheap. Unreadable folders
+// report false.
+func folderHasChildren(fsPath string, foldersOnly bool) bool {
+	f, err := os.Open(fsPath)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	for {
+		batch, err := f.ReadDir(32)
+		for _, e := range batch {
+			name := e.Name()
+			if strings.HasPrefix(name, ".") {
+				continue
+			}
+			if e.IsDir() || (!foldersOnly && name != "index.md") {
+				return true
+			}
+		}
+		if err != nil {
+			return false
+		}
+	}
+}
+
 // treePathSegments splits a wiki path into its non-empty segments.
 func treePathSegments(p string) []string {
 	var segs []string