fix(tui): proper box corners on column ticket area

Replace the full-width ▔▔▔ underline bar with ┌──┐ as the top border of
the ticket box. The ▔ row had no │ side characters so the box corners were
open/disconnected when the ticket area sides started on the next row.
Now the box is closed: ┌──┐ top, │ │ sides, ╘══╛/└──┘ bottom.
The pill name row above it still provides the visual tab header.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

packages/kanban/lib/src/commands/tui_command.dart

@@ -653,9 +653,9 @@ class TuiCommand extends DewCommand {
       bold: isSelected,
     ));
 
-    // Underline bar — ▔ (upper-eighth-block) for selected, thin ─ for inactive
+    // Top border of the ticket box — proper corners so the box closes cleanly
     cells.add(_Cell(
-      isSelected ? '▔' * colW : '─' * colW,
+      '┌${'─' * innerW}┐',
       fg: isSelected ? color : ConsoleColor.brightBlack,
     ));