feat: UI de configuration des colonnes et mappings label→colonne

- Bouton ⚙ sur chaque colonne pour configurer le mapping label Gitea
- Modal ⚙ Colonnes pour ajouter/renommer/supprimer des colonnes
- API DELETE /api/col-mapping pour supprimer un mapping
- Rafraîchit le board après chaque modification de colonne/mapping
- CSS pour les nouveaux éléments (config panel, column list)

static/css/app.css

@@ -119,6 +119,25 @@ body { min-height: 100vh; }
   padding: 10px 12px; display: flex; justify-content: space-between; align-items: center;
   border-bottom: 1px solid var(--border); flex-shrink: 0;
 }
+.column-header-right { display: flex; align-items: center; gap: 6px; }
+.col-config-btn {
+  background: none; border: none; cursor: pointer; font-size: 13px;
+  color: var(--text-dim); padding: 2px 4px; border-radius: 4px;
+  line-height: 1; opacity: 0.5; transition: opacity 0.15s;
+}
+.col-config-btn:hover { opacity: 1; color: var(--accent); }
+.col-config-panel {
+  padding: 8px 12px; border-bottom: 1px solid var(--border);
+  background: var(--bg);
+}
+.col-config-inner { display: flex; flex-direction: column; gap: 6px; }
+.col-config-actions { display: flex; gap: 6px; }
+.config-column-list {
+  list-style: none; display: flex; flex-direction: column; gap: 8px;
+}
+.config-column-row {
+  display: flex; align-items: center; gap: 8px;
+}
 .column-title { font-size: 13px; font-weight: 600; }
 .column-count {
   background: var(--border); border-radius: 10px; padding: 1px 8px;