refactor(file-viewer): remove dead markdown-preview renderer now superseded by the rich editor

Markdown files route exclusively to RichMarkdownEditor on both the read-only
and editable paths, so PreviewPanel's markdown branch and its Streamdown-based
renderer were unreachable. Delete MarkdownPreview and its renderers, callout/
frontmatter/checkbox machinery, and the now-unused remark/rehype/prism/streamdown
imports; drop the dead toggleMarkdownCheckbox/onCheckboxToggle plumbing in
text-editor. Keep the html/csv/svg/mermaid branches intact.

Author: waleedlatif1