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)

app/templates/board_fragment.html

@@ -3,7 +3,22 @@
   <div class="board-column" data-column="{{ col }}" id="col-{{ loop.index }}">
     <div class="column-header">
       <span class="column-title">{{ col }}</span>
-      <span class="column-count">{{ column_map[col]|length }}</span>
+      <div class="column-header-right">
+        <span class="column-count">{{ column_map[col]|length }}</span>
+        <button class="col-config-btn" onclick="toggleColConfig(event, '{{ owner }}','{{ repo }}','{{ col }}')" title="Configurer la colonne">⚙</button>
+      </div>
+    </div>
+    <div class="col-config-panel" id="col-config-{{ col|replace(' ', '_') }}" style="display:none">
+      <div class="col-config-inner">
+        <label class="form-label">Label Gitea → cette colonne</label>
+        <select class="form-select" id="col-label-{{ col|replace(' ', '_') }}">
+          <option value="">— Aucun mapping —</option>
+        </select>
+        <div class="col-config-actions">
+          <button class="btn btn-sm btn-primary" onclick="saveColMapping('{{ owner }}','{{ repo }}','{{ col }}')">Mapper</button>
+          <button class="btn btn-sm" onclick="deleteColMapping('{{ owner }}','{{ repo }}','{{ col }}')">✕ Supprimer</button>
+        </div>
+      </div>
     </div>
     <div class="column-cards sortable" data-column="{{ col }}" data-owner="{{ owner }}" data-repo="{{ repo }}">
       {% for issue in column_map[col] %}