Expanded HTML widgets stay on old snapshots after updates #389

When an agent publishes an updated HTML widget, the version already expanded in the side panel remains on the old snapshot. The user has to expand the new chat widget, creating another panel tab and leaving stale copies alongside the current one.

To reproduce:

1. Publish a page through html_render titled “Progress” showing version 1.
2. Choose Open in panel on the chat widget.
3. Publish a page titled “Progress” showing version 2.
4. Chat shows version 2, while the expanded panel still shows version 1.
5. Expanding version 2 opens a separate panel tab.

Each publish allocates a fresh attachment ID. The panel identifies pages by that attachment ID, and publishing has no stable widget key or update target. Reusing a title does not update the original attachment; reloading it still shows its original HTML. A widget's own data fetching is separate from this publishing behavior.

Support explicitly updating an existing logical widget so its expanded panel follows the latest published content. Preserve the ability to create independent widgets intentionally; titles alone should not define identity.

Deferred: address this after the other side-panel and integrated-browser issues are worked out. This issue records the behavior; it does not expand the scope of the widget-margin fix.