- Current pane gets muted left/right borders only (no full box, no accent
ring); border colors are passed only when a border is requested, since
opentui flips borderless boxes to bordered when borderColor is supplied.
- Remove the Up / <current tab> / Detail titles above the panes; the current
pane's title now renders once, top-left in the parent column's header slot.
- Drop the parentLabel/previewLabel props from PaneRow and all callers.
- Remove the tab/depth indicator from the bottom-left of the status bar.
- Tests measure column widths from the border glyphs and assert the
left/right edges render muted regardless of focus.
Make parent|current|preview fully borderless: no scrollbox borders and no
accent border highlight on the current column. focused still gates
scroll-following but never surfaces a separator. Update tests to measure
column widths from the header-label row and assert no border glyphs render.
