gui: sidebar tree UX fixes — has_children, preserve expanded, double-click, DnD visual

- Backend: add HasChildren field to NodeDTO; ListWorkspaceTree/ListChildren
  populate it by querying CountChildren for container types
- Node repository: add CountChildren(parentID, types...) method
- TreeNode: toggle shown only when has_children (not isContainer); double-click
  on row = expand/collapse; icon click = expand/collapse; drop-valid class via
  DOM classList for DnD highlight; auto-expand collapsed container on 500ms
  hover during drag; auto-scroll near edge during drag
- App.svelte: selectNode no longer resets workspaceTree/expanded; new
  reloadTreePreservingExpanded() helper re-fetches children for all expanded
  nodes after DnD move / delete; deleteWorkspaceNode preserves expanded state

internal/core/nodes/repository.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"path/filepath"
+	"strings"
 	"time"
 
 	"verstak/internal/core/storage"
@@ -168,6 +169,22 @@ func (r *Repository) ListRoots(includeDeleted bool) ([]Node, error) {
 	return scanNodes(rows)
 }
 
+// CountChildren returns the number of non-deleted children for a parent,
+// optionally filtered by one or more types.
+func (r *Repository) CountChildren(parentID string, types ...string) (int, error) {
+	q := `SELECT COUNT(*) FROM nodes WHERE parent_id = ? AND deleted_at IS NULL`
+	args := []interface{}{parentID}
+	if len(types) > 0 {
+		q += " AND type IN (" + strings.Repeat("?,", len(types)-1) + "?)"
+		for _, t := range types {
+			args = append(args, t)
+		}
+	}
+	var count int
+	err := r.db.QueryRow(q, args...).Scan(&count)
+	return count, err
+}
+
 // ListByParent returns children as *Node pointers. parentID must not be empty.
 func (r *Repository) ListByParent(parentID string) ([]*Node, error) {
 	rows, err := r.db.Query(