Fix scene edit not showing or saving HA/OpenHAB entity binding

[jamesmulcahy]

Summary

• When editing an existing scene, the HA/OpenHAB entity selector was never rendered because entity_source and the items list were not passed to the template
• The save handler also ignored backend_name from the POST data when editing, so even if the field had been visible, the selection would not have been saved

Changes

partial_entity_edit_scene_entity() now populates entity_source, backend_name, and the relevant items list (HA or OpenHAB) so the entity selector renders with the current binding pre-selected.

create_or_update_scene_entity() now updates backend_name when saving an existing HA or OpenHAB scene.

Test plan

• Edit an existing Home Assistant scene — confirm the HA entity selector appears with the current binding selected
• Change the HA entity and save — confirm the new binding is persisted
• Edit an existing OpenHAB scene — confirm the same behaviour
• Edit an NSPM-native scene — confirm no entity selector appears (unchanged behaviour)
• Create a new scene — confirm the add flow is unaffected

🤖 Generated with Claude Code

[tpanajott]

I think I'll wait to see if I need to merge this PR or not. As stated in another PR I'm currently redoing a lot of these GUI elements to use React instead as it's really a mess doing it this way. Thanks for the PR anyway.

[jamesmulcahy]

OK, understood -- will hold off on other fixes/tweaks in this area!