fix: boutons telechargement fonctionnels + refonte UI downloads (#17, #8)

- Route GET /api/downloads/video/{task_id} pour streamer les videos
- Route POST /api/downloads/{task_id}/retry pour relancer les failed
- Route POST /api/downloads/cancel-all pour annuler tous les actifs
- Barre de progression animee (shimmer + pulse)
- Indicateurs visuels par status (bordures colorees)
- Bouton Retry pour telechargements echoues/annules
- Actions groupees (Nettoyer termines, Tout annuler)
- Compteur de telechargements actifs
- hx-on::after-request pour refresh auto

Closes #17, Closes #8

static/css/style.css

@@ -646,3 +646,65 @@ h1 {
     from { transform: translateX(100%); opacity: 0; }
     to { transform: translateX(0); opacity: 1; }
 }
+
+/* ==================== Download Items ==================== */
+.download-item {
+    padding: 15px 18px;
+    background: var(--bg-card);
+    border-radius: var(--card-radius);
+    border-left: 4px solid var(--text-dim);
+    margin-bottom: 10px;
+    transition: var(--transition);
+}
+
+.download-item:hover {
+    border-color: var(--primary);
+    transform: translateX(3px);
+}
+
+.download-item.status-downloading {
+    border-left-color: var(--primary);
+    animation: pulse-border 2s ease-in-out infinite;
+}
+
+.download-item.status-completed {
+    border-left-color: var(--accent);
+}
+
+.download-item.status-failed,
+.download-item.status-cancelled {
+    border-left-color: var(--danger);
+}
+
+.download-item.status-paused {
+    border-left-color: #f0a500;
+}
+
+.download-item.status-pending {
+    border-left-color: var(--text-dim);
+}
+
+@keyframes pulse-border {
+    0%, 100% { border-left-color: var(--primary); }
+    50% { border-left-color: rgba(0, 217, 255, 0.3); }
+}
+
+/* Progress bar shimmer */
+.download-item.status-downloading .progress-bar {
+    background: linear-gradient(90deg, var(--primary) 0%, var(--accent) 50%, var(--primary) 100%);
+    background-size: 200% 100%;
+    animation: shimmer 1.5s ease-in-out infinite;
+}
+
+@keyframes shimmer {
+    0% { background-position: 200% 0; }
+    100% { background-position: -200% 0; }
+}
+
+/* Download action buttons */
+.download-actions .btn-icon.warning {
+    color: #f0a500;
+}
+.download-actions .btn-icon.warning:hover {
+    background: rgba(240, 165, 0, 0.2);
+}