Clarify that Web viewport height only affects the designer preview

Rename the Web designer's Viewport height label to Preview viewport height and add a tooltip explaining that the setting only affects the designer preview and does not modify the project.

Suggested tooltip: "Changes only the designer preview height and does not modify the project."

This makes it clear why changing the preview height leaves the project unmodified. Actual layout changes, such as Row Span and Fill remaining, continue to be saved and undoable.