Add real-time upload progress tracking, mobile-optimized manage_group page with player status cards

Features:
- Real-time upload progress tracking with AJAX polling and session-based monitoring
- API endpoint /api/upload_progress/<session_id> for progress updates
- Video conversion progress tracking with background threads
- Mobile-responsive design for manage_group page
- Player status cards with feedback, playlist sync, and last activity
- Bootstrap Icons integration throughout UI
- Responsive layout (1/4 group info, 3/4 players on desktop)
- Video thumbnails with play icon, image thumbnails in media lists
- Bulk selection and delete for group media
- Enhanced logging for video conversion debugging

app/templates/player_page.html

@@ -214,10 +214,22 @@
                                     
                                     <!-- Media Thumbnail and Name -->
                                     <div class="flex-grow-1 mb-2 mb-md-0 d-flex align-items-center">
-                                        <img src="{{ url_for('static', filename='uploads/' ~ media.file_name) }}"
-                                             alt="thumbnail"
-                                             style="width: 48px; height: 48px; object-fit: cover; margin-right: 10px; border-radius: 4px;"
-                                             onerror="this.style.display='none';">
+                                        {% set file_ext = media.file_name.lower().split('.')[-1] %}
+                                        {% if file_ext in ['mp4', 'avi', 'mkv', 'mov', 'webm'] %}
+                                            <!-- Video Icon for video files -->
+                                            <div style="width: 48px; height: 48px; margin-right: 10px; border-radius: 4px; background: linear-gradient(135deg, #667eea 0%, #764ba2 100%); display: flex; align-items: center; justify-content: center;">
+                                                <svg xmlns="http://www.w3.org/2000/svg" width="28" height="28" fill="white" viewBox="0 0 16 16">
+                                                    <path d="M8 15A7 7 0 1 1 8 1a7 7 0 0 1 0 14zm0 1A8 8 0 1 0 8 0a8 8 0 0 0 0 16z"/>
+                                                    <path d="M6.271 5.055a.5.5 0 0 1 .52.038l3.5 2.5a.5.5 0 0 1 0 .814l-3.5 2.5A.5.5 0 0 1 6 10.5v-5a.5.5 0 0 1 .271-.445z"/>
+                                                </svg>
+                                            </div>
+                                        {% else %}
+                                            <!-- Image thumbnail for image files -->
+                                            <img src="{{ url_for('static', filename='uploads/' ~ media.file_name) }}"
+                                                 alt="thumbnail"
+                                                 style="width: 48px; height: 48px; object-fit: cover; margin-right: 10px; border-radius: 4px;"
+                                                 onerror="this.style.display='none';">
+                                        {% endif %}
                                         <p class="mb-0"><strong>Media Name:</strong> {{ media.file_name }}</p>
                                     </div>