fix: Update cleanup button to remove failed, cancelled and completed downloads

Change the 'Nettoyer' (Cleanup) button behavior to remove unwanted downloads:

Before:
- Only removed completed downloads

Now:
- Removes cancelled downloads
- Removes failed downloads
- Removes completed downloads
- Keeps only: downloading, paused, pending downloads
- Shows detailed count by status in confirmation dialog
- Better title tooltip explaining what gets deleted

The confirmation message now shows exactly what will be deleted:
- 'Supprimer X annulé(s) Y échoué(s) Z terminé(s)?'

This makes it easy to clean up the download history while keeping
active downloads safe.

templates/index.html

@@ -812,7 +812,7 @@
             </div>
 
             <div class="actions-group">
-                <button class="btn-small btn-secondary" onclick="clearCompleted()" title="Supprimer les terminés">
+                <button class="btn-small btn-secondary" onclick="clearCompleted()" title="Supprimer annulés, échoués et terminés">
                     <svg fill="none" stroke="currentColor" viewBox="0 0 24 24" style="width:14px;height:14px;">
                         <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16"></path>
                     </svg>
@@ -958,20 +958,37 @@
             return groups;
         }
 
-        // Clear completed downloads
+        // Clear unwanted downloads (cancelled, deleted, failed)
         async function clearCompleted() {
-            const completed = allDownloads.filter(dl => dl.status === 'completed');
+            // Find all unwanted downloads (not completed or downloading or paused)
+            const unwanted = allDownloads.filter(dl =>
+                dl.status === 'cancelled' ||
+                dl.status === 'failed' ||
+                dl.status === 'completed'  // Also remove completed if desired
+            );
 
-            if (completed.length === 0) {
-                alert('Aucun téléchargement terminé à supprimer');
+            if (unwanted.length === 0) {
+                alert('Aucun téléchargement à supprimer');
                 return;
             }
 
-            if (!confirm(`Supprimer ${completed.length} téléchargement(s) terminé(s) ?`)) {
+            // Count by status
+            const byStatus = unwanted.reduce((acc, dl) => {
+                acc[dl.status] = (acc[dl.status] || 0) + 1;
+                return acc;
+            }, {});
+
+            let message = 'Supprimer ';
+            if (byStatus.cancelled) message += `${byStatus.cancelled} annulé(s) `;
+            if (byStatus.failed) message += `${byStatus.failed} échoué(s) `;
+            if (byStatus.completed) message += `${byStatus.completed} terminé(s) `;
+            message += '?';
+
+            if (!confirm(message)) {
                 return;
             }
 
-            for (const dl of completed) {
+            for (const dl of unwanted) {
                 await fetch(`${API_BASE}/download/${dl.id}`, { method: 'DELETE' });
             }