Add leftover media management, PDF to PNG conversion, video delete, and playlist duration editing

Features added:
- Leftover media management page in admin panel with delete functionality for images and videos
- Individual file delete buttons for leftover media
- PDF to PNG conversion (each page becomes a separate image at Full HD resolution)
- Delete functionality for leftover video files
- Enhanced playlist management with duration editing for all content types
- Improved dark mode support for playlist management page
- Content type badges with color coding
- Better styling for duration input fields with save functionality
- Fixed URL generation for duration update endpoint

app/blueprints/admin.py

@@ -490,3 +490,29 @@ def delete_leftover_videos():
         flash(f'Error deleting leftover videos: {str(e)}', 'danger')
     
     return redirect(url_for('admin.leftover_media'))
+
+@admin_bp.route('/delete-single-leftover/<int:content_id>', methods=['POST'])
+@login_required
+@admin_required
+def delete_single_leftover(content_id):
+    """Delete a single leftover content file"""
+    try:
+        content = Content.query.get_or_404(content_id)
+        
+        # Delete physical file
+        if content.file_path:
+            file_path = os.path.join(current_app.config['UPLOAD_FOLDER'], content.file_path)
+            if os.path.exists(file_path):
+                os.remove(file_path)
+        
+        # Delete database record
+        db.session.delete(content)
+        db.session.commit()
+        
+        flash(f'Successfully deleted {content.file_path}', 'success')
+        
+    except Exception as e:
+        db.session.rollback()
+        flash(f'Error deleting file: {str(e)}', 'danger')
+    
+    return redirect(url_for('admin.leftover_media'))