Add a bright accent-coloured 1-column bar on the inner left edge
of the active panel. This is a premium TUI pattern (also used by
bottom, lazygit, btop) that gives a much clearer visual focus
indicator than a border colour change alone.
The bar uses the same active_border_color as the panel border so
it stays thematically consistent. The panel inner content shifts
right by one column to accommodate the bar without losing content.
Tests: 1388 pass, zero warnings.
vasilito
dc32c6ccbf