Subjects editor UX follow-ups: schema placement and badge, deep-tree layout, relation edit-button ambiguity

[JeroenDeDauw]

Collected follow-ups from the 2026-08-31 review of #1323; none blocks its merge. One issue on purpose — split anything out that grows.

• Schema placement and display. Move the schema up beside the label so the header is one compact row (the editor is wider now), and unify how a subject's schema is shown — currently a link in the header and subtle text next to tree nodes — into one form, e.g. a badge that always means "schema". That also lets the header schema tag go, and should reduce the misclick where the label pencil gets hit when the user means to edit the schema (the one edit icon near the top attracts the click; a one-row layout probably fixes it, not certain).
• Deep trees. Allow horizontal scrolling within the tree pane and/or make the divider between tree and editor pane adjustable.
• Relation edit-button ambiguity. The per-relation edit button edits the target subject; someone wanting to change which subject is linked may click it and edit the wrong subject's data instead of changing the link. Accepted for now as a fail-once cost — a two-button picker was judged worse — recorded here in case testing shows otherwise.

_{AI-authored — Claude Code, Fable 5 (max); filed on @JeroenDeDauw's go-ahead from the 2026-08-31 sync follow-up list, bundling chosen by him; text not human-reviewed; content checked against PR 1323 and the meeting record.}

[JeroenDeDauw]

The edit pencil next to the label also keeps confusing me, as in I keep clicking it automatically when I want to edit the schema

I just did it again after mentioning it in the meeting earlier. And I know what each UI element does. Still incorrectly clicking the pencil.