feat(20-03): kanban board, task card, etapes section, files table CSS + task-list gap
- .tablo-kanban-board flex container with overflow-x auto
- .tablo-kanban-column 18rem wide with border-radius 0.75rem
- .tablo-kanban-column-header muted surface background
- .tablo-kanban-column-title, -task-count, -add-link, -empty rules
- .task-card column-flex with hover shadow and border-color transition
- .task-drag-handle opacity 0 at rest, opacity 1 on .task-card:hover
- .task-card-delete opacity 0 at rest, opacity 1 on .task-card:hover
- .task-card-top-row, .task-card-title helpers
- .task-list updated with gap 8px and padding 8px
- .tablo-etapes-section, .tablo-etape-row, .tablo-etape-name, .tablo-etape-count
- .tablo-files-table-wrapper with border-radius 12px and table header styles
- .task-row and .tasks-section unchanged (no CSS cascade regression)