MarkdownViewer images now load from web and local paths

MarkdownViewer now displays both online and local Markdown images reliably. HTTP and HTTPS image URLs load asynchronously, relative local paths resolve against BasePath, and changing BasePath refreshes existing Markdown automatically. Downloaded images are remeasured after they arrive, so they render at their intended size rather than retaining the alternative-text placeholder height. The bundled MarkdownViewer Demo now illustrates both a project-resource image and an online image.