Dynamic, non-blocking loading for inline images in emails

Previously, a single skeleton loader covered the entire email details view and only disappeared after all inline images finished loading. For emails with many large images, this could take 20–30 seconds. Now each image loads independently, and the Forward action opens instantly by reusing the same image loading state. Images smaller than 24×24 px skip the skeleton loader, and images with broken URLs show the browser's default placeholder. Skeleton loaders will match final image size 1:1 for newly composed emails and forwarded messages, since those have width and height attributes set automatically.