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)

go-backend/internal/web/ui/app.css

@@ -1283,6 +1283,8 @@ td.text-right .borderless-icon-button.ui-icon-button-ghost.ui-icon-button-danger
 .task-list {
   display: flex;
   flex-direction: column;
+  gap: 8px;
+  padding: 8px;
 }
 
 .task-row {
@@ -2032,3 +2034,190 @@ td.text-right .borderless-icon-button.ui-icon-button-ghost.ui-icon-button-danger
   font-size: 0.875rem;
   padding-bottom: 12px;
 }
+
+.tablo-kanban-board {
+  display: flex;
+  gap: 16px;
+  overflow-x: auto;
+  padding-bottom: 16px;
+  padding-top: 24px;
+}
+
+.tablo-kanban-column {
+  background: var(--color-surface-default);
+  border: 1px solid var(--color-border-default);
+  border-radius: 0.75rem;
+  flex-shrink: 0;
+  overflow: hidden;
+  width: 18rem;
+}
+
+.tablo-kanban-column-header {
+  align-items: center;
+  background: var(--color-surface-muted);
+  display: flex;
+  gap: 8px;
+  justify-content: space-between;
+  padding: 12px 16px;
+}
+
+.tablo-kanban-column-title {
+  color: var(--color-text-primary);
+  font-size: 0.875rem;
+  font-weight: 600;
+  margin: 0;
+}
+
+.tablo-kanban-task-count {
+  background: var(--color-surface-default);
+  border: 1px solid var(--color-border-muted);
+  border-radius: 9999px;
+  color: var(--color-text-secondary);
+  font-size: 0.75rem;
+  padding: 0 8px;
+}
+
+.tablo-kanban-add-link {
+  color: var(--color-text-brand);
+  font-size: 0.875rem;
+  font-weight: 400;
+  margin-left: auto;
+  text-decoration: none;
+}
+
+.tablo-kanban-empty {
+  color: var(--color-text-faint);
+  font-size: 0.875rem;
+  padding: 24px 16px;
+  text-align: center;
+}
+
+.task-card {
+  background: var(--color-surface-default);
+  border: 1px solid var(--color-border-default);
+  border-radius: 8px;
+  cursor: pointer;
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  padding-block: 8px;
+  padding-inline: 12px;
+  transition: box-shadow 0.12s ease, border-color 0.12s ease;
+}
+
+.task-card:hover {
+  border-color: var(--color-border-strong);
+  box-shadow: 0 8px 24px rgba(15, 23, 42, 0.08);
+}
+
+.task-card-top-row {
+  align-items: flex-start;
+  display: flex;
+  gap: 6px;
+}
+
+.task-drag-handle {
+  color: var(--color-text-faint);
+  cursor: grab;
+  flex-shrink: 0;
+  font-size: 1rem;
+  opacity: 0;
+  transition: opacity 0.12s ease;
+}
+
+.task-card:hover .task-drag-handle {
+  opacity: 1;
+}
+
+.task-card-title {
+  color: var(--color-text-primary);
+  flex: 1;
+  font-size: 0.875rem;
+  font-weight: 400;
+  line-height: 1.4;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.task-card-delete {
+  flex-shrink: 0;
+  opacity: 0;
+  transition: opacity 0.12s ease;
+}
+
+.task-card:hover .task-card-delete {
+  opacity: 1;
+}
+
+.tablo-etapes-section {
+  border-top: 1px solid var(--color-border-muted);
+  margin-top: 24px;
+  padding-top: 24px;
+}
+
+.tablo-etapes-section h2,
+.tablo-etapes-section h3 {
+  color: var(--color-text-primary);
+  font-size: 1rem;
+  font-weight: 600;
+  margin: 0 0 12px 0;
+}
+
+.tablo-etapes-section ul {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.tablo-etape-row {
+  align-items: center;
+  background: var(--color-surface-default);
+  border: 1px solid var(--color-border-default);
+  border-radius: 8px;
+  display: flex;
+  gap: 12px;
+  justify-content: space-between;
+  padding: 10px 16px;
+}
+
+.tablo-etape-name {
+  color: var(--color-text-primary);
+  font-size: 0.875rem;
+  font-weight: 500;
+}
+
+.tablo-etape-count {
+  color: var(--color-text-secondary);
+  font-size: 0.75rem;
+}
+
+.tablo-files-table-wrapper {
+  border: 1px solid var(--color-border-default);
+  border-radius: 12px;
+  overflow: hidden;
+}
+
+.tablo-files-table-wrapper thead tr {
+  background: var(--color-surface-muted);
+}
+
+.tablo-files-table-wrapper thead th {
+  color: var(--color-text-muted);
+  font-size: 0.75rem;
+  font-weight: 600;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+}
+
+.tablo-files-table-wrapper tbody tr {
+  border-bottom: 1px solid var(--color-border-default);
+}
+
+.tablo-files-table-wrapper tbody tr:hover {
+  background: var(--color-surface-subtle);
+}