[BYOM] Use of placeholder attributes on fields is confusing

[jonathansampson]

Description

When creating or editing a BYOM model configuration, the UI presently shows the default context size and default system prompt in the respective input elements. The values are, however, presented via the placeholder attribute for each field. This presentation may look to the end-user like the content is editable, which is not the case. If the user prefers to keep the default prompt, but with a couple changes, they are not able to do so. Any input by the user completely overwrites the placeholder text.

[BrendanEich]

We shouldn't use a readonly-looking style for anything editable, but beyond style, it is odd that one cannot select (visibly) but can type to overwrite. I've never seen this style and half-joked about "HIG compliance" (old Apple folks will grok) to Sampson.