Apply styles across labels from the node/edge style dialogs

## Description

In the node/edge style dialogs there's no way to reuse a label's custom styles on another label — each label must be styled from scratch.

## Preferred Solution

Add actions to apply a label's custom styles to other labels. This could manifest as "copy styles from label", "apply styles to label", both, or other options.

## Related Issues

- Part of #1896

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.

No files, tests, or entry points are named. Start by locating the node/edge style dialogs and their label-style state, then determine how users should copy or apply custom styles across labels; done means the chosen style action works in both dialogs.

Written by the indexing model from the issue text.