fix(sections-editor): stop Add variant button overlapping field controls

[vitoUwu]

Summary

In the site editor's section properties panel, the multivariate Add variant flag button was absolutely positioned at right-0 top-0 of the field it wraps (multivariate-field-wrapper.tsx).

That's empty space for the common stacked field (label above input), but BooleanField is the one field with an inline layout — flex items-center justify-between with the Switch pinned right — so the flag rendered directly on top of the toggle. It covered the switch, stole clicks aimed at its right half, and popped the "Add variant" tooltip over the control on hover.

Fix: give the button its own gutter column (grid-cols-[minmax(0,1fr)_auto] items-start) instead of overlaying the field. The button keeps its visual position (top-right, aligned with the label row thanks to items-start) and now also renders after the field in the DOM, so focus order is input → Add variant rather than the reverse.

Trade-off: fields that can be made multivariate lose ~32px of width to the gutter. That's the cost of not covering controls; the alternative — keeping the overlay and special-casing boolean via a layout hint threaded from schema-form.tsx — is more code and breaks again for the next inline-layout field.

Testing

• bun run fmt, bun run --cwd=apps/web check, bun run lint (0 errors; the 14 warnings are pre-existing and none are in the touched file).
• Verified manually in the sections editor: the "Use Simple Search" toggle on Global Header is no longer covered, and the flag button still wraps the field into a variant.

No existing test asserted the overlay.

🤖 Generated with Claude Code

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Summary by cubic

Stops the "Add variant" button in the sections editor's property panel from overlapping field controls, so boolean toggles are no longer covered or have their clicks stolen.

The button now sits in its own gutter column instead of overlaying the field and renders after the field, so focus order is input → Add variant. Fields that can be multivariate lose ~32px of width to the gutter.

^{Written for commit c093378. Summary will update on new commits.}