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/core/nodes/repository.go

@@ -4,9 +4,11 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"path/filepath"
 	"time"
 
 	"verstak/internal/core/storage"
+	"verstak/internal/core/templates"
 	"verstak/internal/core/util"
 )
 
@@ -264,7 +266,8 @@ func (r *Repository) UpdateFsPathRecursive(id, newFsPath string) error {
 		return err
 	}
 	for _, child := range children {
-		childPath := newFsPath + "/" + child.Slug
+		seg := templates.SafeDisplayNameToPathSegment(child.Title)
+		childPath := filepath.Join(newFsPath, seg)
 		if err := r.UpdateFsPathRecursive(child.ID, childPath); err != nil {
 			return err
 		}