Restore center-pane borders, move title to top-left slot

- 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.

src/pages/Player/PlayerPage.tsx

@@ -119,7 +119,6 @@ export function PlayerPage() {
 		<PaneRow
 			parent={parentContent}
 			current={currentContent}
-			parentLabel="Up"
 			currentLabel="Player"
 			panes={2}
 			focused={isActive}