Add background upload management feature

- Add BackgroundUploadManager class for managing uploads in background
- Create background upload button and panel UI
- Implement upload state transfer when modal closes
- Add real-time progress tracking for background uploads
- Support up to 5 concurrent uploads
- Display upload progress with percentage and chunk info
- Auto-hide background UI when all uploads complete
- Add responsive design for background panel

static/style.css

@@ -224,6 +224,178 @@ body {
     background: #d32f2f;
 }
 
+.btn-info {
+    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+    border: none;
+    color: white;
+}
+
+.btn-info:hover:not(:disabled) {
+    background: linear-gradient(135deg, #5a67d8 0%, #6b46c1 100%);
+}
+
+.background-btn {
+    animation: pulse 2s infinite;
+    font-size: 13px;
+    padding: 6px 12px;
+    white-space: nowrap;
+}
+
+@keyframes pulse {
+    0% { transform: scale(1); }
+    50% { transform: scale(1.02); }
+    100% { transform: scale(1); }
+}
+
+.background-panel {
+    position: fixed;
+    bottom: 20px;
+    right: 20px;
+    width: 350px;
+    max-height: 400px;
+    background: white;
+    border-radius: 8px;
+    box-shadow: 0 4px 20px rgba(0,0,0,0.15);
+    z-index: 1000;
+    display: none;
+    flex-direction: column;
+}
+
+.panel-header {
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+    padding: 12px 16px;
+    border-bottom: 1px solid #f0f0f0;
+    background: #f8f9fa;
+    border-radius: 8px 8px 0 0;
+}
+
+.panel-header h4 {
+    margin: 0;
+    font-size: 14px;
+    color: #333;
+    font-weight: 600;
+}
+
+.close-btn {
+    background: none;
+    border: none;
+    font-size: 18px;
+    color: #999;
+    cursor: pointer;
+    padding: 0;
+    width: 24px;
+    height: 24px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    border-radius: 4px;
+}
+
+.close-btn:hover {
+    background: #e9ecef;
+    color: #666;
+}
+
+.background-content {
+    flex: 1;
+    overflow: hidden;
+    display: flex;
+    flex-direction: column;
+}
+
+.background-list {
+    flex: 1;
+    overflow-y: auto;
+    max-height: 350px;
+}
+
+.empty-state {
+    padding: 40px 20px;
+    text-align: center;
+    color: #999;
+    font-size: 14px;
+}
+
+.background-item {
+    display: flex;
+    align-items: center;
+    padding: 10px;
+    border-bottom: 1px solid #f0f0f0;
+    transition: background-color 0.2s;
+}
+
+.background-item:hover {
+    background: #f8f9fa;
+}
+
+.background-item:last-child {
+    border-bottom: none;
+}
+
+.item-icon {
+    font-size: 16px;
+    width: 24px;
+    text-align: center;
+    flex-shrink: 0;
+}
+
+.item-info {
+    flex: 1;
+    min-width: 0;
+    margin-left: 10px;
+}
+
+.item-name {
+    font-size: 12px;
+    color: #333;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    margin-bottom: 4px;
+    font-weight: 500;
+}
+
+.item-progress {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+}
+
+.progress-bar {
+    width: 80px;
+    height: 4px;
+    background: #e0e0e0;
+    border-radius: 2px;
+    overflow: hidden;
+    flex-shrink: 0;
+}
+
+.progress-fill {
+    height: 100%;
+    background: #2196f3;
+    border-radius: 2px;
+    transition: width 0.3s ease;
+}
+
+.progress-text {
+    font-size: 11px;
+    color: #666;
+    font-family: monospace;
+    min-width: 60px;
+}
+
+.item-status {
+    width: 20px;
+    text-align: center;
+    flex-shrink: 0;
+}
+
+.status-icon {
+    font-size: 14px;
+}
+
 .file-list-container {
     background: #fff;
     border-radius: 6px;