Now users can free to edit their custom models by double clicking any
custom models created in each provider.
Before this, they have to delete -> create a new one.
I simply add an edit panel (which looks the same as 'Add Custom Model')
and integrate that process into the "update" button.

There is one more issue that if a user deletes a model that he was using
in chat, then back to chat, that model would still appear (and work)
unless user chooses a new one.
Tried to modify "delete-custom-model" in language_model_handlers.ts by
the logic that if the name of that model matches the latest using one ->
switch to auto (or default) model. Yet I failed, maybe need more
explanation for this :)